fix: avoid side effect on plugin options

src/index.js

@@ -32,16 +32,17 @@ const NAME = 'rollup-plugin-prettier';
 
 module.exports = (options) => {
   let sourcemap = null;
+  let newOptions = options;
 
   if (options && hasSourceMap(options)) {
     sourcemap = isSourceMapEnabled(options);
 
     // Delete custom option.
-    deleteSourceMap(options);
+    newOptions = deleteSourceMap(options);
 
     // Do not send an empty option object.
-    if (Object.keys(options).length === 0) {
-      options = undefined;
+    if (Object.keys(newOptions).length === 0) {
+      newOptions = undefined;
     }
   }
 
@@ -83,7 +84,7 @@ module.exports = (options) => {
      * @return {Object} The result containing a `code` property and, if a enabled, a `map` property.
      */
     transformBundle(source, outputOptions) {
-      const output = prettier.format(source, options);
+      const output = prettier.format(source, newOptions);
       const outputOptionsSourcemap = isNil(outputOptions) ? null : isSourceMapEnabled(outputOptions);
 
       // Should we generate sourcemap?
@@ -165,9 +166,18 @@ function isSourceMapEnabled(opts) {
  * Delete sourcemap option on object.
  *
  * @param {Object} opts The object.
+ * @return {Object} Option object without `sourcemap` entry.
  */
 function deleteSourceMap(opts) {
-  SOURCE_MAPS_OPTS.forEach((p) => delete opts[p]);
+  const newOptions = {};
+
+  Object.keys(opts).forEach((k) => {
+    if (SOURCE_MAPS_OPTS.indexOf(k) < 0) {
+      newOptions[k] = opts[k];
+    }
+  });
+
+  return newOptions;
 }
 
 /**

test/index.spec.js

@@ -24,6 +24,7 @@
 
 'use strict';
 
+const prettier = require('prettier');
 const plugin = require('../dist/index.js');
 
 describe('rollup-plugin-prettier', () => {
@@ -232,4 +233,60 @@ describe('rollup-plugin-prettier', () => {
       'var test = "hello world";\n'
     );
   });
+
+  it('should avoid side effect and do not modify plugin options', () => {
+    const options = {
+      sourceMap: false,
+    };
+
+    const instance = plugin(options);
+    instance.options({});
+    instance.transformBundle('var foo = 0;');
+
+    // It should not have been touched.
+    expect(options).toEqual({
+      sourceMap: false,
+    });
+  });
+
+  it('should run prettier without sourcemap options', () => {
+    const options = {
+      sourceMap: false,
+    };
+
+    spyOn(prettier, 'format').and.callThrough();
+
+    const code = 'var foo = 0;';
+    const instance = plugin(options);
+    instance.options({});
+    instance.transformBundle(code);
+
+    expect(prettier.format).toHaveBeenCalledWith(code, undefined);
+    expect(options).toEqual({
+      sourceMap: false,
+    });
+  });
+
+  it('should run prettier without sourcemap options and custom other options', () => {
+    const options = {
+      sourceMap: false,
+      singleQuote: true,
+    };
+
+    spyOn(prettier, 'format').and.callThrough();
+
+    const code = 'var foo = 0;';
+    const instance = plugin(options);
+    instance.options({});
+    instance.transformBundle(code);
+
+    expect(prettier.format).toHaveBeenCalledWith(code, {
+      singleQuote: true,
+    });
+
+    expect(options).toEqual({
+      sourceMap: false,
+      singleQuote: true,
+    });
+  });
 });