diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..9fe93ed
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,15 @@
+; EditorConfig file: https://EditorConfig.org
+; Install the "EditorConfig" plugin into your editor to use
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+
+[package.json]
+indent_size = 2
diff --git a/.eslintrc.cjs b/.eslintrc.cjs
new file mode 100644
index 0000000..3b591b1
--- /dev/null
+++ b/.eslintrc.cjs
@@ -0,0 +1,30 @@
+"use strict";
+
+module.exports = {
+    root: true,
+    extends: [
+        "eslint",
+        "plugin:node/recommended-module"
+    ],
+    parserOptions: {
+        sourceType: "module",
+        ecmaVersion: "2020"
+    },
+    overrides: [
+        {
+            files: ["tests/**/*"],
+            env: {
+                mocha: true
+            }
+        },
+        {
+            files: ["*.cjs"],
+            extends: ["eslint", "plugin:node/recommended-script"],
+            parserOptions: {
+                sourceType: "script",
+                ecmaVersion: "2020"
+            }
+        }
+    ],
+    ignorePatterns: ["/dist", "/coverage"]
+};
diff --git a/.eslintrc.js b/.eslintrc.js
deleted file mode 100644
index bc0737e..0000000
--- a/.eslintrc.js
+++ /dev/null
@@ -1,16 +0,0 @@
-"use strict";
-
-module.exports = {
-    root: true,
-    extends: [
-        "eslint"
-    ],
-    overrides: [
-        {
-            files: ["tests/**/*"],
-            env: {
-                mocha: true
-            }
-        }
-    ]
-};
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d0a5f6f..7ed8d02 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,7 +25,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        node: [14.x, 13.x, 12.x, 10.x, 8.x]
+        node: [16.x, 14.x, 12.x, "12.22.0"]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
diff --git a/.gitignore b/.gitignore
index 3222b4e..f191c4c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,4 @@ npm-debug.log
 /.vscode
 .sublimelinterrc
 .eslint-release-info.json
+dist
diff --git a/Makefile.js b/Makefile.js
index 952691c..31b2c95 100644
--- a/Makefile.js
+++ b/Makefile.js
@@ -6,24 +6,29 @@
  */
 /* global echo, exec, exit, set, target */
 
-"use strict";
-
 /* eslint no-console: 0*/
 //------------------------------------------------------------------------------
 // Requirements
 //------------------------------------------------------------------------------
 
-require("shelljs/make");
-set("+e");
+import path from "path";
+import { fileURLToPath } from "url";
+
+import "shelljs/make.js";
+import checker from "npm-license";
+
+const dirname = path.dirname(fileURLToPath(import.meta.url));
 
-const checker = require("npm-license");
+// `shelljs/make.js` global command to unset any `set('-e')` (to exit upon
+//    first error)
+set("+e");
 
 //------------------------------------------------------------------------------
 // Settings
 //------------------------------------------------------------------------------
 
 const OPEN_SOURCE_LICENSES = [
-    /MIT/, /BSD/, /Apache/, /ISC/, /WTF/, /Public Domain/
+    /MIT/u, /BSD/u, /Apache/u, /ISC/u, /WTF/u, /Public Domain/u
 ];
 
 //------------------------------------------------------------------------------
@@ -31,17 +36,21 @@ const OPEN_SOURCE_LICENSES = [
 //------------------------------------------------------------------------------
 
 const NODE = "node",
-    NODE_MODULES = "./node_modules/",
+    NODE_MODULES = "./node_modules",
 
     // Utilities - intentional extra space at the end of each string
-    MOCHA = `${NODE_MODULES}mocha/bin/_mocha `,
+    MOCHA = `${NODE_MODULES}/mocha/bin/_mocha `,
     ESLINT = `${NODE} ${NODE_MODULES}/eslint/bin/eslint `,
-    ISTANBUL = `${NODE} ${NODE_MODULES}/istanbul/lib/cli.js `,
+
+    // If switching back to Istanbul when may be working with ESM
+    // ISTANBUL = `${NODE} ${NODE_MODULES}/istanbul/lib/cli.js `,
+    C8 = `${NODE} ${NODE_MODULES}/c8/bin/c8.js`,
 
     // Files
     MAKEFILE = "./Makefile.js",
     JS_FILES = "lib/**/*.js",
-    TEST_FILES = "tests/*.js";
+    TEST_FILES = "tests/*.js",
+    CJS_TEST_FILES = "tests/*.cjs";
 
 //------------------------------------------------------------------------------
 // Tasks
@@ -73,6 +82,12 @@ target.lint = function() {
         errors++;
     }
 
+    echo("Validating CJS JavaScript test files");
+    lastReturn = exec(ESLINT + CJS_TEST_FILES);
+    if (lastReturn.code !== 0) {
+        errors++;
+    }
+
     if (errors) {
         exit(1);
     }
@@ -80,7 +95,13 @@ target.lint = function() {
 
 target.test = function() {
     let errors = 0;
-    const lastReturn = exec(`${ISTANBUL} cover ${MOCHA} -- -R progress -c ${TEST_FILES}`);
+    let lastReturn = exec(`${NODE} ${MOCHA} -- -R progress -c ${CJS_TEST_FILES}`);
+
+    if (lastReturn.code !== 0) {
+        errors++;
+    }
+
+    lastReturn = exec(`${C8} ${MOCHA} -- -R progress -c ${TEST_FILES}`);
 
     if (lastReturn.code !== 0) {
         errors++;
@@ -116,7 +137,7 @@ target.checkLicenses = function() {
     echo("Validating licenses");
 
     checker.init({
-        start: __dirname
+        start: dirname
     }, deps => {
         const impermissible = Object.keys(deps).map(dependency => ({
             name: dependency,
diff --git a/README.md b/README.md
index e824b42..4ad41d5 100644
--- a/README.md
+++ b/README.md
@@ -2,35 +2,47 @@
 
 ESLint Scope is the [ECMAScript](http://www.ecma-international.org/publications/standards/Ecma-262.htm) scope analyzer used in ESLint. It is a fork of [escope](http://github.com/estools/escope).
 
-## Usage
-
-Install:
+## Install
 
 ```
 npm i eslint-scope --save
 ```
 
+## 📖 Usage
+
+To use in an ESM file:
+
+```js
+import * as eslintScope from 'eslint-scope';
+```
+
+To use in a CommonJS file:
+
+```js
+const eslintScope = require('eslint-scope');
+```
+
 Example:
 
 ```js
-var eslintScope = require('eslint-scope');
-var espree = require('espree');
-var estraverse = require('estraverse');
+import * as eslintScope from 'eslint-scope';
+import * as espree from 'espree';
+import estraverse from 'estraverse';
 
-var ast = espree.parse(code, { range: true });
-var scopeManager = eslintScope.analyze(ast);
+const ast = espree.parse(code, { range: true });
+const scopeManager = eslintScope.analyze(ast);
 
-var currentScope = scopeManager.acquire(ast);   // global scope
+const currentScope = scopeManager.acquire(ast);   // global scope
 
 estraverse.traverse(ast, {
-    enter: function(node, parent) {
+    enter (node, parent) {
         // do stuff
 
         if (/Function/.test(node.type)) {
             currentScope = scopeManager.acquire(node);  // get current function scope
         }
     },
-    leave: function(node, parent) {
+    leave(node, parent) {
         if (/Function/.test(node.type)) {
             currentScope = currentScope.upper;  // set to parent scope
         }
diff --git a/lib/definition.js b/lib/definition.js
index 172bfe2..9744ef4 100644
--- a/lib/definition.js
+++ b/lib/definition.js
@@ -21,18 +21,17 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
-const Variable = require("./variable");
+import Variable from "./variable.js";
 
 /**
- * @class Definition
+ * @constructor Definition
  */
 class Definition {
     constructor(type, name, node, parent, index, kind) {
 
         /**
-         * @member {String} Definition#type - type of the occurrence (e.g. "Parameter", "Variable", ...).
+         * @member {string} Definition#type - type of the occurrence (e.g. "Parameter", "Variable", ...).
          */
         this.type = type;
 
@@ -52,19 +51,19 @@ class Definition {
         this.parent = parent;
 
         /**
-         * @member {Number?} Definition#index - the index in the declaration statement.
+         * @member {number?} Definition#index - the index in the declaration statement.
          */
         this.index = index;
 
         /**
-         * @member {String?} Definition#kind - the kind of the declaration statement.
+         * @member {string?} Definition#kind - the kind of the declaration statement.
          */
         this.kind = kind;
     }
 }
 
 /**
- * @class ParameterDefinition
+ * @constructor ParameterDefinition
  */
 class ParameterDefinition extends Definition {
     constructor(name, node, index, rest) {
@@ -78,7 +77,7 @@ class ParameterDefinition extends Definition {
     }
 }
 
-module.exports = {
+export {
     ParameterDefinition,
     Definition
 };
diff --git a/lib/index.js b/lib/index.js
index 0f16fa4..ac386ad 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -45,18 +45,16 @@
  * The main interface is the {@link analyze} function.
  * @module escope
  */
-"use strict";
-
 /* eslint no-underscore-dangle: ["error", { "allow": ["__currentScope"] }] */
 
-const assert = require("assert");
+import assert from "assert";
+
+import ScopeManager from "./scope-manager.js";
+import Referencer from "./referencer.js";
+import Reference from "./reference.js";
+import Variable from "./variable.js";
 
-const ScopeManager = require("./scope-manager");
-const Referencer = require("./referencer");
-const Reference = require("./reference");
-const Variable = require("./variable");
-const Scope = require("./scope").Scope;
-const version = require("../package.json").version;
+import eslintScopeVersion from "./version.js";
 
 /**
  * Set the default options
@@ -77,15 +75,15 @@ function defaultOptions() {
 
 /**
  * Preform deep update on option object
- * @param {Object} target - Options
- * @param {Object} override - Updates
+ * @param {Object} target Options
+ * @param {Object} override Updates
  * @returns {Object} Updated options
  */
 function updateDeeply(target, override) {
 
     /**
      * Is hash object
-     * @param {Object} value - Test value
+     * @param {Object} value Test value
      * @returns {boolean} Result
      */
     function isHashObject(value) {
@@ -114,20 +112,20 @@ function updateDeeply(target, override) {
  * Main interface function. Takes an Espree syntax tree and returns the
  * analyzed scopes.
  * @function analyze
- * @param {espree.Tree} tree - Abstract Syntax Tree
- * @param {Object} providedOptions - Options that tailor the scope analysis
- * @param {boolean} [providedOptions.optimistic=false] - the optimistic flag
- * @param {boolean} [providedOptions.directive=false]- the directive flag
- * @param {boolean} [providedOptions.ignoreEval=false]- whether to check 'eval()' calls
- * @param {boolean} [providedOptions.nodejsScope=false]- whether the whole
+ * @param {espree.Tree} tree Abstract Syntax Tree
+ * @param {Object} providedOptions Options that tailor the scope analysis
+ * @param {boolean} [providedOptions.optimistic=false] the optimistic flag
+ * @param {boolean} [providedOptions.directive=false] the directive flag
+ * @param {boolean} [providedOptions.ignoreEval=false] whether to check 'eval()' calls
+ * @param {boolean} [providedOptions.nodejsScope=false] whether the whole
  * script is executed under node.js environment. When enabled, escope adds
  * a function scope immediately following the global scope.
- * @param {boolean} [providedOptions.impliedStrict=false]- implied strict mode
+ * @param {boolean} [providedOptions.impliedStrict=false] implied strict mode
  * (if ecmaVersion >= 5).
- * @param {string} [providedOptions.sourceType='script']- the source type of the script. one of 'script' and 'module'
- * @param {number} [providedOptions.ecmaVersion=5]- which ECMAScript version is considered
- * @param {Object} [providedOptions.childVisitorKeys=null] - Additional known visitor keys. See [esrecurse](https://github.com/estools/esrecurse)'s the `childVisitorKeys` option.
- * @param {string} [providedOptions.fallback='iteration'] - A kind of the fallback in order to encounter with unknown node. See [esrecurse](https://github.com/estools/esrecurse)'s the `fallback` option.
+ * @param {string} [providedOptions.sourceType='script'] the source type of the script. one of 'script' and 'module'
+ * @param {number} [providedOptions.ecmaVersion=5] which ECMAScript version is considered
+ * @param {Object} [providedOptions.childVisitorKeys=null] Additional known visitor keys. See [esrecurse](https://github.com/estools/esrecurse)'s the `childVisitorKeys` option.
+ * @param {string} [providedOptions.fallback='iteration'] A kind of the fallback in order to encounter with unknown node. See [esrecurse](https://github.com/estools/esrecurse)'s the `fallback` option.
  * @returns {ScopeManager} ScopeManager
  */
 function analyze(tree, providedOptions) {
@@ -142,10 +140,10 @@ function analyze(tree, providedOptions) {
     return scopeManager;
 }
 
-module.exports = {
+export {
 
     /** @name module:escope.version */
-    version,
+    eslintScopeVersion as version,
 
     /** @name module:escope.Reference */
     Reference,
@@ -153,13 +151,22 @@ module.exports = {
     /** @name module:escope.Variable */
     Variable,
 
-    /** @name module:escope.Scope */
-    Scope,
-
     /** @name module:escope.ScopeManager */
     ScopeManager,
+
+    /** @name module:escope.Referencer */
+    Referencer,
+
     analyze
 };
 
+/** @name module:escope.Definition */
+export { Definition } from "./definition.js";
+
+/** @name module:escope.PatternVisitor */
+export { default as PatternVisitor } from "./pattern-visitor.js";
+
+/** @name module:escope.Scope */
+export { Scope } from "./scope.js";
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/pattern-visitor.js b/lib/pattern-visitor.js
index afa6291..a9ff48e 100644
--- a/lib/pattern-visitor.js
+++ b/lib/pattern-visitor.js
@@ -21,16 +21,17 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 /* eslint-disable no-undefined */
 
-const Syntax = require("estraverse").Syntax;
-const esrecurse = require("esrecurse");
+import estraverse from "estraverse";
+import esrecurse from "esrecurse";
+
+const { Syntax } = estraverse;
 
 /**
  * Get last array element
- * @param {array} xs - array
+ * @param {Array} xs array
  * @returns {any} Last elment
  */
 function getLast(xs) {
@@ -147,6 +148,6 @@ class PatternVisitor extends esrecurse.Visitor {
     }
 }
 
-module.exports = PatternVisitor;
+export default PatternVisitor;
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/reference.js b/lib/reference.js
index 9529827..e657d62 100644
--- a/lib/reference.js
+++ b/lib/reference.js
@@ -21,7 +21,6 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 const READ = 0x1;
 const WRITE = 0x2;
@@ -29,7 +28,7 @@ const RW = READ | WRITE;
 
 /**
  * A Reference represents a single occurrence of an identifier in code.
- * @class Reference
+ * @constructor Reference
  */
 class Reference {
     constructor(ident, scope, flag, writeExpr, maybeImplicitGlobal, partial, init) {
@@ -91,7 +90,7 @@ class Reference {
 
     /**
      * Whether the reference is static.
-     * @method Reference#isStatic
+     * @function Reference#isStatic
      * @returns {boolean} static
      */
     isStatic() {
@@ -100,7 +99,7 @@ class Reference {
 
     /**
      * Whether the reference is writeable.
-     * @method Reference#isWrite
+     * @function Reference#isWrite
      * @returns {boolean} write
      */
     isWrite() {
@@ -109,7 +108,7 @@ class Reference {
 
     /**
      * Whether the reference is readable.
-     * @method Reference#isRead
+     * @function Reference#isRead
      * @returns {boolean} read
      */
     isRead() {
@@ -118,7 +117,7 @@ class Reference {
 
     /**
      * Whether the reference is read-only.
-     * @method Reference#isReadOnly
+     * @function Reference#isReadOnly
      * @returns {boolean} read only
      */
     isReadOnly() {
@@ -127,7 +126,7 @@ class Reference {
 
     /**
      * Whether the reference is write-only.
-     * @method Reference#isWriteOnly
+     * @function Reference#isWriteOnly
      * @returns {boolean} write only
      */
     isWriteOnly() {
@@ -136,7 +135,7 @@ class Reference {
 
     /**
      * Whether the reference is read-write.
-     * @method Reference#isReadWrite
+     * @function Reference#isReadWrite
      * @returns {boolean} read write
      */
     isReadWrite() {
@@ -162,6 +161,6 @@ Reference.WRITE = WRITE;
  */
 Reference.RW = RW;
 
-module.exports = Reference;
+export default Reference;
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/referencer.js b/lib/referencer.js
index 7a79118..0b90ec8 100644
--- a/lib/referencer.js
+++ b/lib/referencer.js
@@ -21,28 +21,26 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 /* eslint-disable no-underscore-dangle */
 /* eslint-disable no-undefined */
 
-const Syntax = require("estraverse").Syntax;
-const esrecurse = require("esrecurse");
-const Reference = require("./reference");
-const Variable = require("./variable");
-const PatternVisitor = require("./pattern-visitor");
-const definition = require("./definition");
-const assert = require("assert");
+import estraverse from "estraverse";
+import esrecurse from "esrecurse";
+import Reference from "./reference.js";
+import Variable from "./variable.js";
+import PatternVisitor from "./pattern-visitor.js";
+import { Definition, ParameterDefinition } from "./definition.js";
+import assert from "assert";
 
-const ParameterDefinition = definition.ParameterDefinition;
-const Definition = definition.Definition;
+const { Syntax } = estraverse;
 
 /**
  * Traverse identifier in pattern
- * @param {Object} options - options
- * @param {pattern} rootPattern - root pattern
- * @param {Refencer} referencer - referencer
- * @param {callback} callback - callback
+ * @param {Object} options options
+ * @param {pattern} rootPattern root pattern
+ * @param {Refencer} referencer referencer
+ * @param {callback} callback callback
  * @returns {void}
  */
 function traverseIdentifierInPattern(options, rootPattern, referencer, callback) {
@@ -209,8 +207,8 @@ class Referencer extends esrecurse.Visitor {
 
         /**
          * Visit pattern callback
-         * @param {pattern} pattern - pattern
-         * @param {Object} info - info
+         * @param {pattern} pattern pattern
+         * @param {Object} info info
          * @returns {void}
          */
         function visitPatternCallback(pattern, info) {
@@ -643,6 +641,6 @@ class Referencer extends esrecurse.Visitor {
     }
 }
 
-module.exports = Referencer;
+export default Referencer;
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/scope-manager.js b/lib/scope-manager.js
index ef56170..5a12314 100644
--- a/lib/scope-manager.js
+++ b/lib/scope-manager.js
@@ -21,11 +21,10 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 /* eslint-disable no-underscore-dangle */
 
-const {
+import {
     BlockScope,
     CatchScope,
     ClassFieldInitializerScope,
@@ -37,11 +36,11 @@ const {
     ModuleScope,
     SwitchScope,
     WithScope
-} = require("./scope");
-const assert = require("assert");
+} from "./scope.js";
+import assert from "assert";
 
 /**
- * @class ScopeManager
+ * @constructor ScopeManager
  */
 class ScopeManager {
     constructor(options) {
@@ -92,8 +91,7 @@ class ScopeManager {
      * "are declared by the node" means the node is same as `Variable.defs[].node` or `Variable.defs[].parent`.
      * If the node declares nothing, this method returns an empty array.
      * CAUTION: This API is experimental. See https://github.com/estools/escope/pull/69 for more details.
-     *
-     * @param {Espree.Node} node - a node to get.
+     * @param {Espree.Node} node a node to get.
      * @returns {Variable[]} variables that declared by the node.
      */
     getDeclaredVariables(node) {
@@ -102,16 +100,16 @@ class ScopeManager {
 
     /**
      * acquire scope from node.
-     * @method ScopeManager#acquire
-     * @param {Espree.Node} node - node for the acquired scope.
-     * @param {boolean=} inner - look up the most inner scope, default value is false.
+     * @function ScopeManager#acquire
+     * @param {Espree.Node} node node for the acquired scope.
+     * @param {?boolean} [inner=false] look up the most inner scope, default value is false.
      * @returns {Scope?} Scope from node
      */
     acquire(node, inner) {
 
         /**
          * predicate
-         * @param {Scope} testScope - scope to test
+         * @param {Scope} testScope scope to test
          * @returns {boolean} predicate
          */
         function predicate(testScope) {
@@ -156,8 +154,8 @@ class ScopeManager {
 
     /**
      * acquire all scopes from node.
-     * @method ScopeManager#acquireAll
-     * @param {Espree.Node} node - node for the acquired scope.
+     * @function ScopeManager#acquireAll
+     * @param {Espree.Node} node node for the acquired scope.
      * @returns {Scopes?} Scope array
      */
     acquireAll(node) {
@@ -166,9 +164,9 @@ class ScopeManager {
 
     /**
      * release the node.
-     * @method ScopeManager#release
-     * @param {Espree.Node} node - releasing node.
-     * @param {boolean=} inner - look up the most inner scope, default value is false.
+     * @function ScopeManager#release
+     * @param {Espree.Node} node releasing node.
+     * @param {?boolean} [inner=false] look up the most inner scope, default value is false.
      * @returns {Scope?} upper scope for the node.
      */
     release(node, inner) {
@@ -247,6 +245,6 @@ class ScopeManager {
     }
 }
 
-module.exports = ScopeManager;
+export default ScopeManager;
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/scope.js b/lib/scope.js
index 651f56a..73386e6 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -21,24 +21,25 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 /* eslint-disable no-underscore-dangle */
 /* eslint-disable no-undefined */
 
-const Syntax = require("estraverse").Syntax;
+import estraverse from "estraverse";
 
-const Reference = require("./reference");
-const Variable = require("./variable");
-const Definition = require("./definition").Definition;
-const assert = require("assert");
+import Reference from "./reference.js";
+import Variable from "./variable.js";
+import { Definition } from "./definition.js";
+import assert from "assert";
+
+const { Syntax } = estraverse;
 
 /**
  * Test if scope is struct
- * @param {Scope} scope - scope
- * @param {Block} block - block
- * @param {boolean} isMethodDefinition - is method definition
- * @param {boolean} useDirective - use directive
+ * @param {Scope} scope scope
+ * @param {Block} block block
+ * @param {boolean} isMethodDefinition is method definition
+ * @param {boolean} useDirective use directive
  * @returns {boolean} is strict scope
  */
 function isStrictScope(scope, block, isMethodDefinition, useDirective) {
@@ -121,8 +122,8 @@ function isStrictScope(scope, block, isMethodDefinition, useDirective) {
 
 /**
  * Register scope
- * @param {ScopeManager} scopeManager - scope manager
- * @param {Scope} scope - scope
+ * @param {ScopeManager} scopeManager scope manager
+ * @param {Scope} scope scope
  * @returns {void}
  */
 function registerScope(scopeManager, scope) {
@@ -139,7 +140,7 @@ function registerScope(scopeManager, scope) {
 
 /**
  * Should be statically
- * @param {Object} def - def
+ * @param {Object} def def
  * @returns {boolean} should be statically
  */
 function shouldBeStatically(def) {
@@ -150,14 +151,14 @@ function shouldBeStatically(def) {
 }
 
 /**
- * @class Scope
+ * @constructor Scope
  */
 class Scope {
     constructor(scopeManager, type, upperScope, block, isMethodDefinition) {
 
         /**
          * One of 'module', 'block', 'switch', 'function', 'catch', 'with', 'function', 'class', 'global'.
-         * @member {String} Scope#type
+         * @member {string} Scope#type
          */
         this.type = type;
 
@@ -461,8 +462,8 @@ class Scope {
 
     /**
      * returns resolved {Reference}
-     * @method Scope#resolve
-     * @param {Espree.Identifier} ident - identifier to be resolved.
+     * @function Scope#resolve
+     * @param {Espree.Identifier} ident identifier to be resolved.
      * @returns {Reference} reference
      */
     resolve(ident) {
@@ -481,7 +482,7 @@ class Scope {
 
     /**
      * returns this scope is static
-     * @method Scope#isStatic
+     * @function Scope#isStatic
      * @returns {boolean} static
      */
     isStatic() {
@@ -490,7 +491,7 @@ class Scope {
 
     /**
      * returns this scope has materialized arguments
-     * @method Scope#isArgumentsMaterialized
+     * @function Scope#isArgumentsMaterialized
      * @returns {boolean} arguemnts materialized
      */
     isArgumentsMaterialized() { // eslint-disable-line class-methods-use-this
@@ -499,7 +500,7 @@ class Scope {
 
     /**
      * returns this scope has materialized `this` reference
-     * @method Scope#isThisMaterialized
+     * @function Scope#isThisMaterialized
      * @returns {boolean} this materialized
      */
     isThisMaterialized() { // eslint-disable-line class-methods-use-this
@@ -527,10 +528,10 @@ class GlobalScope extends Scope {
             variables: [],
 
             /**
-            * List of {@link Reference}s that are left to be resolved (i.e. which
-            * need to be linked to the variable they refer to).
-            * @member {Reference[]} Scope#implicit#left
-            */
+             * List of {@link Reference}s that are left to be resolved (i.e. which
+             * need to be linked to the variable they refer to).
+             * @member {Reference[]} Scope#implicit#left
+             */
             left: []
         };
     }
@@ -737,7 +738,7 @@ class ClassFieldInitializerScope extends Scope {
     }
 }
 
-module.exports = {
+export {
     Scope,
     GlobalScope,
     ModuleScope,
diff --git a/lib/variable.js b/lib/variable.js
index 702c478..286202f 100644
--- a/lib/variable.js
+++ b/lib/variable.js
@@ -21,19 +21,18 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
 /**
  * A Variable represents a locally scoped identifier. These include arguments to
  * functions.
- * @class Variable
+ * @constructor Variable
  */
 class Variable {
     constructor(name, scope) {
 
         /**
          * The variable name, as given in the source code.
-         * @member {String} Variable#name
+         * @member {string} Variable#name
          */
         this.name = name;
 
@@ -83,6 +82,6 @@ Variable.Variable = "Variable";
 Variable.ImportBinding = "ImportBinding";
 Variable.ImplicitGlobalVariable = "ImplicitGlobalVariable";
 
-module.exports = Variable;
+export default Variable;
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/lib/version.js b/lib/version.js
new file mode 100644
index 0000000..aa53e2a
--- /dev/null
+++ b/lib/version.js
@@ -0,0 +1,3 @@
+const version = "5.1.1";
+
+export default version;
diff --git a/package.json b/package.json
index b1739df..65c3ddb 100644
--- a/package.json
+++ b/package.json
@@ -2,10 +2,18 @@
   "name": "eslint-scope",
   "description": "ECMAScript scope analyzer for ESLint",
   "homepage": "http://github.com/eslint/eslint-scope",
-  "main": "lib/index.js",
+  "main": "./dist/eslint-scope.cjs",
+  "type": "module",
+  "exports": {
+    ".": {
+      "import": "./lib/index.js",
+      "require": "./dist/eslint-scope.cjs"
+    },
+    "./package.json": "./package.json"
+  },
   "version": "5.1.1",
   "engines": {
-    "node": ">=8.0.0"
+    "node": "^12.22.0 || ^14.17.0 || >=16.0.0"
   },
   "repository": "eslint/eslint-scope",
   "bugs": {
@@ -13,6 +21,9 @@
   },
   "license": "BSD-2-Clause",
   "scripts": {
+    "prepare": "npm run update-version && npm run build",
+    "build": "rollup -c",
+    "update-version": "node tools/update-version.js",
     "test": "node Makefile.js test",
     "lint": "node Makefile.js lint",
     "generate-release": "eslint-generate-release",
@@ -24,25 +35,28 @@
   "files": [
     "LICENSE",
     "README.md",
-    "lib"
+    "lib",
+    "dist/eslint-scope.cjs"
   ],
   "dependencies": {
     "esrecurse": "^4.3.0",
     "estraverse": "^5.2.0"
   },
   "devDependencies": {
-    "@typescript-eslint/parser": "^1.11.0",
-    "chai": "^4.2.0",
-    "eslint": "^6.0.1",
-    "eslint-config-eslint": "^5.0.1",
-    "eslint-plugin-node": "^9.1.0",
-    "eslint-release": "^1.0.0",
+    "@typescript-eslint/parser": "^4.28.1",
+    "c8": "^7.7.3",
+    "chai": "^4.3.4",
+    "eslint": "^7.29.0",
+    "eslint-config-eslint": "^7.0.0",
+    "eslint-plugin-jsdoc": "^35.4.1",
+    "eslint-plugin-node": "^11.1.0",
+    "eslint-release": "^3.1.2",
     "eslint-visitor-keys": "^3.0.0",
     "espree": "^8.0.0",
-    "istanbul": "^0.4.5",
-    "mocha": "^6.1.4",
+    "mocha": "^9.0.1",
     "npm-license": "^0.3.3",
-    "shelljs": "^0.8.3",
-    "typescript": "^3.5.2"
+    "rollup": "^2.52.7",
+    "shelljs": "^0.8.4",
+    "typescript": "^4.3.5"
   }
 }
diff --git a/rollup.config.js b/rollup.config.js
new file mode 100644
index 0000000..135e147
--- /dev/null
+++ b/rollup.config.js
@@ -0,0 +1,10 @@
+export default {
+    input: "./lib/index.js",
+    external: ["assert", "estraverse", "esrecurse"],
+    treeshake: false,
+    output: {
+        format: "cjs",
+        file: "dist/eslint-scope.cjs",
+        sourcemap: true
+    }
+};
diff --git a/tests/arguments.js b/tests/arguments.js
index 60038d7..268f210 100644
--- a/tests/arguments.js
+++ b/tests/arguments.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("arguments", () => {
     it("arguments are correctly materialized", () => {
diff --git a/tests/catch-scope.js b/tests/catch-scope.js
index 62c52d1..e27f7f6 100644
--- a/tests/catch-scope.js
+++ b/tests/catch-scope.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("catch", () => {
     it("creates scope", () => {
diff --git a/tests/child-visitor-keys.js b/tests/child-visitor-keys.js
index 8ff0be5..d700424 100644
--- a/tests/child-visitor-keys.js
+++ b/tests/child-visitor-keys.js
@@ -20,11 +20,10 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("childVisitorKeys option", () => {
     it("should handle as a known node if the childVisitorKeys option was given.", () => {
diff --git a/tests/class-fields.js b/tests/class-fields.js
index a8739aa..57714d8 100644
--- a/tests/class-fields.js
+++ b/tests/class-fields.js
@@ -3,12 +3,10 @@
  * @author Toru Nagashima
  */
 
-"use strict";
-
-const assert = require("assert");
-const espree = require("espree");
-const { KEYS } = require("eslint-visitor-keys");
-const { analyze } = require("../lib/index");
+import assert from "assert";
+import * as espree from "espree";
+import { KEYS } from "eslint-visitor-keys";
+import { analyze } from "../lib/index.js";
 
 describe("Class fields", () => {
     describe("class C { f = g }", () => {
diff --git a/tests/commonjs.cjs b/tests/commonjs.cjs
new file mode 100644
index 0000000..23f17df
--- /dev/null
+++ b/tests/commonjs.cjs
@@ -0,0 +1,41 @@
+/**
+ * @fileoverview Tests for checking that the commonjs entry points are still accessible
+ * @author Mike Reinstein
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const assert = require("assert");
+const eslintScope = require("../dist/eslint-scope.cjs");
+
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+describe("commonjs", () => {
+    it("is an object", () => {
+        assert.strictEqual(typeof eslintScope, "object");
+    });
+
+    it("has exports", () => {
+        assert.strictEqual(typeof eslintScope.version, "string");
+
+        [
+            "analyze",
+            "Definition",
+            "PatternVisitor",
+            "Reference",
+            "Referencer",
+            "Scope",
+            "ScopeManager",
+            "Variable"
+        ].forEach(prop => {
+            assert.strictEqual(typeof eslintScope[prop], "function");
+        });
+    });
+});
diff --git a/tests/es6-arrow-function-expression.js b/tests/es6-arrow-function-expression.js
index 998ef73..87beafc 100644
--- a/tests/es6-arrow-function-expression.js
+++ b/tests/es6-arrow-function-expression.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 arrow function expression", () => {
     it("materialize scope for arrow function expression", () => {
diff --git a/tests/es6-block-scope.js b/tests/es6-block-scope.js
index 1e5a21e..8ddf5bc 100644
--- a/tests/es6-block-scope.js
+++ b/tests/es6-block-scope.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 block scope", () => {
     it("let is materialized in ES6 block scope#1", () => {
diff --git a/tests/es6-catch.js b/tests/es6-catch.js
index afc24e9..26419f3 100644
--- a/tests/es6-catch.js
+++ b/tests/es6-catch.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 catch", () => {
     it("takes binding pattern", () => {
diff --git a/tests/es6-class.js b/tests/es6-class.js
index 93c5620..0816458 100644
--- a/tests/es6-class.js
+++ b/tests/es6-class.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 class", () => {
     it("declaration name creates class scope", () => {
diff --git a/tests/es6-default-parameters.js b/tests/es6-default-parameters.js
index d88adc2..ef7f28d 100644
--- a/tests/es6-default-parameters.js
+++ b/tests/es6-default-parameters.js
@@ -20,14 +20,13 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 /* eslint-disable guard-for-in */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 default parameters:", () => {
     describe("a default parameter creates a writable reference for its initialization:", () => {
diff --git a/tests/es6-destructuring-assignments.js b/tests/es6-destructuring-assignments.js
index 89fa664..9a85808 100644
--- a/tests/es6-destructuring-assignments.js
+++ b/tests/es6-destructuring-assignments.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 destructuring assignments", () => {
     it("Pattern in var in ForInStatement", () => {
diff --git a/tests/es6-export.js b/tests/es6-export.js
index d015b42..b683f62 100644
--- a/tests/es6-export.js
+++ b/tests/es6-export.js
@@ -20,11 +20,10 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("export declaration", () => {
 
diff --git a/tests/es6-import.js b/tests/es6-import.js
index 6a03886..0cd8e9d 100644
--- a/tests/es6-import.js
+++ b/tests/es6-import.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("import declaration", () => {
 
diff --git a/tests/es6-iteration-scope.js b/tests/es6-iteration-scope.js
index fb478e6..2ed630f 100644
--- a/tests/es6-iteration-scope.js
+++ b/tests/es6-iteration-scope.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 iteration scope", () => {
     it("let materialize iteration scope for ForInStatement#1", () => {
diff --git a/tests/es6-new-target.js b/tests/es6-new-target.js
index 16d5cdd..dc10c58 100644
--- a/tests/es6-new-target.js
+++ b/tests/es6-new-target.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 new.target", () => {
     it("should not make references of new.target", () => {
diff --git a/tests/es6-object.js b/tests/es6-object.js
index 8f9e700..e94967d 100644
--- a/tests/es6-object.js
+++ b/tests/es6-object.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 object", () => {
     it("method definition", () => {
diff --git a/tests/es6-rest-args.js b/tests/es6-rest-args.js
index 637f1f2..36dc61e 100644
--- a/tests/es6-rest-args.js
+++ b/tests/es6-rest-args.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 rest arguments", () => {
     it("materialize rest argument in scope", () => {
diff --git a/tests/es6-super.js b/tests/es6-super.js
index 7a74edf..dc42e2e 100644
--- a/tests/es6-super.js
+++ b/tests/es6-super.js
@@ -20,11 +20,10 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 super", () => {
     it("is not handled as reference", () => {
diff --git a/tests/es6-switch.js b/tests/es6-switch.js
index 780a6e1..05be10c 100644
--- a/tests/es6-switch.js
+++ b/tests/es6-switch.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 switch", () => {
     it("materialize scope", () => {
diff --git a/tests/es6-template-literal.js b/tests/es6-template-literal.js
index 8b1f4ec..d72a5bb 100644
--- a/tests/es6-template-literal.js
+++ b/tests/es6-template-literal.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("ES6 template literal", () => {
     it("refer variables", () => {
diff --git a/tests/export-star-as-ns-from-source.js b/tests/export-star-as-ns-from-source.js
index 49d4257..85c98d8 100644
--- a/tests/export-star-as-ns-from-source.js
+++ b/tests/export-star-as-ns-from-source.js
@@ -1,9 +1,7 @@
-"use strict";
-
-const assert = require("assert");
-const espree = require("espree");
-const { KEYS } = require("eslint-visitor-keys");
-const { analyze } = require("../lib/index");
+import assert from "assert";
+import * as espree from "espree";
+import { KEYS } from "eslint-visitor-keys";
+import { analyze } from "../lib/index.js";
 
 describe("export * as ns from 'source'", () => {
     let scopes;
diff --git a/tests/fallback.js b/tests/fallback.js
index 6c7dde5..cfbc6a3 100644
--- a/tests/fallback.js
+++ b/tests/fallback.js
@@ -20,12 +20,10 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
-
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
 
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("fallback option", () => {
     it("should raise an error when it encountered an unknown node if no fallback.", () => {
diff --git a/tests/function-expression-name.js b/tests/function-expression-name.js
index f982b83..ef238d8 100644
--- a/tests/function-expression-name.js
+++ b/tests/function-expression-name.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("function name", () => {
     it("should create its special scope", () => {
diff --git a/tests/get-declared-variables.js b/tests/get-declared-variables.js
index ee42289..48b54eb 100644
--- a/tests/get-declared-variables.js
+++ b/tests/get-declared-variables.js
@@ -20,15 +20,16 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const visit = require("esrecurse").visit;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import esrecurse from "esrecurse";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
+
+const { visit } = esrecurse;
 
 describe("ScopeManager.prototype.getDeclaredVariables", () => {
-    /* eslint-disable require-jsdoc */
+    /* eslint-disable jsdoc/require-jsdoc */
     function verify(ast, type, expectedNamesList) {
         const scopeManager = analyze(ast, {
             ecmaVersion: 6,
diff --git a/tests/global-increment.js b/tests/global-increment.js
index df48b94..53517d9 100644
--- a/tests/global-increment.js
+++ b/tests/global-increment.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("global increment", () => {
     it("becomes read/write", () => {
diff --git a/tests/implicit-global-reference.js b/tests/implicit-global-reference.js
index 011a5f8..4c869ff 100644
--- a/tests/implicit-global-reference.js
+++ b/tests/implicit-global-reference.js
@@ -19,11 +19,10 @@
 // ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 // THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("implicit global reference", () => {
     it("assignments global scope", () => {
diff --git a/tests/implied-strict.js b/tests/implied-strict.js
index 3896941..2d07869 100644
--- a/tests/implied-strict.js
+++ b/tests/implied-strict.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("impliedStrict option", () => {
     it("ensures all user scopes are strict if ecmaVersion >= 5", () => {
diff --git a/tests/label.js b/tests/label.js
index 1605a23..f3330b3 100644
--- a/tests/label.js
+++ b/tests/label.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("label", () => {
     it("should not create variables", () => {
diff --git a/tests/nodejs-scope.js b/tests/nodejs-scope.js
index 2ce8cf7..c3a90ca 100644
--- a/tests/nodejs-scope.js
+++ b/tests/nodejs-scope.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("nodejsScope option", () => {
     it("creates a function scope following the global scope immediately", () => {
diff --git a/tests/object-expression.js b/tests/object-expression.js
index 81b4a9d..f3440d7 100644
--- a/tests/object-expression.js
+++ b/tests/object-expression.js
@@ -1,7 +1,5 @@
-"use strict";
-
-const expect = require("chai").expect;
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import { analyze } from "../lib/index.js";
 
 describe("object expression", () => {
     it("doesn't require property type", () => {
diff --git a/tests/optimistic.js b/tests/optimistic.js
index ddf0d06..941e724 100644
--- a/tests/optimistic.js
+++ b/tests/optimistic.js
@@ -19,11 +19,10 @@
 // ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 // THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("optimistic", () => {
     it("direct call to eval", () => {
diff --git a/tests/references.js b/tests/references.js
index 6de0a1b..b9b4213 100644
--- a/tests/references.js
+++ b/tests/references.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("References:", () => {
     describe("When there is a `let` declaration on global,", () => {
diff --git a/tests/typescript.js b/tests/typescript.js
index 0b312b3..8ac53c2 100644
--- a/tests/typescript.js
+++ b/tests/typescript.js
@@ -2,7 +2,6 @@
  * @fileoverview Typescript scope tests
  * @author Reyad Attiyat
  */
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
@@ -10,9 +9,11 @@
 // Requirements
 //------------------------------------------------------------------------------
 
-const expect = require("chai").expect,
-    parse = require("@typescript-eslint/parser").parse,
-    analyze = require("..").analyze;
+import { expect } from "chai";
+import typescriptEslintParser from "@typescript-eslint/parser";
+import { analyze } from "../lib/index.js";
+
+const { parse } = typescriptEslintParser;
 
 //------------------------------------------------------------------------------
 // Tests
@@ -27,7 +28,9 @@ describe("typescript", () => {
                 function foo(bar: string | number): string | number {
                     return bar;
                 }
-            `);
+            `, {
+                range: true
+            });
 
             const scopeManager = analyze(ast);
 
diff --git a/tests/util/espree.js b/tests/util/espree.js
index 6804513..3e3de7a 100644
--- a/tests/util/espree.js
+++ b/tests/util/espree.js
@@ -21,18 +21,21 @@
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-"use strict";
 
-const espree = require("espree");
-
-module.exports = function(code, sourceType) {
-    sourceType = sourceType || "module";
+import * as espree from "espree";
 
+/**
+ * Parse into Espree AST.
+ * @param {string} code The code
+ * @param {"module"|"script"} [sourceType="module"] The source type
+ * @returns {Object} The parsed Espree AST
+ */
+export default function(code, sourceType = "module") {
     return espree.parse(code, {
         range: true,
         ecmaVersion: 7,
         sourceType
     });
-};
+}
 
 /* vim: set sw=4 ts=4 et tw=80 : */
diff --git a/tests/with-scope.js b/tests/with-scope.js
index 2d33e3d..f90bd33 100644
--- a/tests/with-scope.js
+++ b/tests/with-scope.js
@@ -20,13 +20,12 @@
 //  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 //  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 //  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-"use strict";
 
 /* eslint-disable no-unused-expressions */
 
-const expect = require("chai").expect;
-const espree = require("./util/espree");
-const analyze = require("..").analyze;
+import { expect } from "chai";
+import espree from "./util/espree.js";
+import { analyze } from "../lib/index.js";
 
 describe("with", () => {
     it("creates scope", () => {
diff --git a/tools/update-version.js b/tools/update-version.js
new file mode 100644
index 0000000..977bcaf
--- /dev/null
+++ b/tools/update-version.js
@@ -0,0 +1,18 @@
+/**
+ * @fileoverview Script to update lib/version.js to the value in package.json
+ * @author Nicholas C. Zakas
+ */
+
+import fs from "fs";
+
+/*
+ * IMPORTANT: This must be run *before* Rollup so the built package will have
+ * the correct version number exported.
+ *
+ * This is necessary because ESM can't import JSON files directly and we want
+ * this value to be available in the browser as well as in Node.js.
+ */
+
+const pkg = JSON.parse(fs.readFileSync("./package.json", "utf8"));
+
+fs.writeFileSync("lib/version.js", `const version = "${pkg.version}";\n\nexport default version;\n`);