fix #1623: ignore class fields marked "abstract"

CHANGELOG.md

@@ -2,6 +2,26 @@
 
 ## Unreleased
 
+* Fix compilation of abstract class fields in TypeScript ([#1623](https://github.com/evanw/esbuild/issues/1623))
+
+    This release fixes a bug where esbuild could incorrectly include a TypeScript abstract class field in the compiled JavaScript output. This is incorrect because the official TypeScript compiler never does this. Note that this only happened in scenarios where TypeScript's `useDefineForClassFields` setting was set to `true` (or equivalently where TypeScript's `target` setting was set to `ESNext`). Here is the difference:
+
+    ```js
+    // Original code
+    abstract class Foo {
+      abstract foo: any;
+    }
+
+    // Old output
+    class Foo {
+      foo;
+    }
+
+    // New output
+    class Foo {
+    }
+    ```
+
 * Proxy from the `__require` shim to `require` ([#1614](https://github.com/evanw/esbuild/issues/1614))
 
     Some background: esbuild's bundler emulates a CommonJS environment. The bundling process replaces the literal syntax `require(<string>)` with the referenced module at compile-time. However, other uses of `require` such as `require(someFunction())` are not bundled since the value of `someFunction()` depends on code evaluation, and esbuild does not evaluate code at compile-time. So it's possible for some references to `require` to remain after bundling.

internal/bundler/bundler_ts_test.go

@@ -1194,3 +1194,53 @@ func TestTSComputedClassFieldUseDefineTrueLower(t *testing.T) {
 		},
 	})
 }
+
+func TestTSAbstractClassFieldUseAssign(t *testing.T) {
+	default_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.ts": `
+				const keepThis = Symbol('keepThis')
+				declare const AND_REMOVE_THIS: unique symbol
+				abstract class Foo {
+					REMOVE_THIS: any
+					[keepThis]: any
+					abstract REMOVE_THIS_TOO: any
+					abstract [AND_REMOVE_THIS]: any
+					abstract [(x => y => x + y)('nested')('scopes')]: any
+				}
+				(() => new Foo())()
+			`,
+		},
+		entryPaths: []string{"/entry.ts"},
+		options: config.Options{
+			Mode:                    config.ModePassThrough,
+			AbsOutputFile:           "/out.js",
+			UseDefineForClassFields: config.False,
+		},
+	})
+}
+
+func TestTSAbstractClassFieldUseDefine(t *testing.T) {
+	default_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.ts": `
+				const keepThisToo = Symbol('keepThisToo')
+				declare const REMOVE_THIS_TOO: unique symbol
+				abstract class Foo {
+					keepThis: any
+					[keepThisToo]: any
+					abstract REMOVE_THIS: any
+					abstract [REMOVE_THIS_TOO]: any
+					abstract [(x => y => x + y)('nested')('scopes')]: any
+				}
+				(() => new Foo())()
+			`,
+		},
+		entryPaths: []string{"/entry.ts"},
+		options: config.Options{
+			Mode:                    config.ModePassThrough,
+			AbsOutputFile:           "/out.js",
+			UseDefineForClassFields: config.True,
+		},
+	})
+}

internal/bundler/snapshots/snapshots_default.txt

@@ -2579,6 +2579,25 @@ switch (bar) {
     let a;
 }
 
+================================================================================
+TestTSAbstractClassFieldUseAssign
+---------- /out.js ----------
+const keepThis = Symbol("keepThis");
+class Foo {
+}
+keepThis;
+(() => new Foo())();
+
+================================================================================
+TestTSAbstractClassFieldUseDefine
+---------- /out.js ----------
+const keepThisToo = Symbol("keepThisToo");
+class Foo {
+  keepThis;
+  [keepThisToo];
+}
+(() => new Foo())();
+
 ================================================================================
 TestTSComputedClassFieldUseDefineFalse
 ---------- /out.js ----------

internal/js_parser/js_parser.go

@@ -1809,6 +1809,7 @@ type propertyOpts struct {
 	// Class-related options
 	isStatic          bool
 	isTSDeclare       bool
+	isTSAbstract      bool
 	isClass           bool
 	classHasExtends   bool
 	allowTSDecorators bool
@@ -1936,7 +1937,16 @@ func (p *parser) parseProperty(kind js_ast.PropertyKind, opts propertyOpts, erro
 						return js_ast.Property{}, false
 					}
 
-				case "private", "protected", "public", "readonly", "abstract", "override":
+				case "abstract":
+					if opts.isClass && p.options.ts.Parse && !opts.isTSAbstract && raw == name {
+						opts.isTSAbstract = true
+						scopeIndex := len(p.scopesInOrder)
+						p.parseProperty(kind, opts, nil)
+						p.discardScopesUpTo(scopeIndex)
+						return js_ast.Property{}, false
+					}
+
+				case "private", "protected", "public", "readonly", "override":
 					// Skip over TypeScript keywords
 					if opts.isClass && p.options.ts.Parse && raw == name {
 						return p.parseProperty(kind, opts, nil)