feat(watch): add skipWrite option (#3454)

* feat(watch): add `skipWrite` option;

- Closes #3225

* chore(docs): add `skipWrite` doc mentions

* chore: add `skipWrite` test

Co-authored-by: Lukas Taegert-Atkinson <lukastaegert@users.noreply.github.com>

docs/01-command-line-reference.md

@@ -100,6 +100,7 @@ export default { // can be an array (for multiple inputs)
   watch: {
     chokidar,
     clearScreen,
+    skipWrite,
     exclude,
     include
   }
@@ -272,20 +273,20 @@ Print the help document.
 Use the specified plugin. There are several ways to specify plugins here:
 
 - Via a relative path:
-  
+
   ```
   rollup -i input.js -f es -p ./my-plugin.js
   ```
-  
+
   The file should export a plugin object or a function returning such an object.
 - Via the name of a plugin that is installed in a local or global `node_modules` folder:
-  
+
   ```
   rollup -i input.js -f es -p @rollup/plugin-node-resolve
   ```
-  
+
   If the plugin name does not start with `rollup-plugin-` or `@rollup/plugin-`, Rollup will automatically try adding these prefixes:
-  
+
   ```
   rollup -i input.js -f es -p node-resolve
   ```
@@ -295,7 +296,7 @@ Use the specified plugin. There are several ways to specify plugins here:
   ```
   rollup -i input.js -f es -p '{transform: (c, i) => `/* ${JSON.stringify(i)} */\n${c}`}'
   ```
-  
+
 If you want to load more than one plugin, you can repeat the option or supply a comma-separated list of names:
 
 ```

docs/02-javascript-api.md

@@ -182,6 +182,7 @@ const watchOptions = {
   watch: {
     chokidar,
     clearScreen,
+    skipWrite,
     exclude,
     include
   }

docs/999-big-list-of-options.md

@@ -361,10 +361,10 @@ export default {
   onwarn (warning, warn) {
     // skip certain warnings
     if (warning.code === 'UNUSED_EXTERNAL_IMPORT') return;
-  
+
     // throw on others
     if (warning.code === 'NON_EXISTENT_EXPORT') throw new Error(warning.message);
-  
+
     // Use default for everything else
     warn(warning);
   }
@@ -754,7 +754,7 @@ Type: `boolean`<br>
 CLI: `--externalLiveBindings`/`--no-externalLiveBindings`<br>
 Default: `true`
 
-When set to `false`, Rollup will not generate code to support live bindings for external imports but instead assume that exports do not change over time. This will enable Rollup to generate more optimized code. Note that this can cause issues when there are circular dependencies involving an external dependency. 
+When set to `false`, Rollup will not generate code to support live bindings for external imports but instead assume that exports do not change over time. This will enable Rollup to generate more optimized code. Note that this can cause issues when there are circular dependencies involving an external dependency.
 
 This will avoid most cases where Rollup generates getters in the code and can therefore be used to make code IE8 compatible in many cases.
 
@@ -1005,11 +1005,11 @@ function test(callback) {
   	// calls to otherwise side-effect-free global functions are retained
   	// inside try-statements for tryCatchDeoptimization: true
     Object.create(null);
-    
+
   	// calls to other function are retained as well but the body of this
   	// function may again be subject to tree-shaking
     otherFn();
-    
+
     // if a parameter is called, then all arguments passed to that function
     // parameter will be deoptimized
     callback();
@@ -1096,6 +1096,13 @@ Default: `true`
 
 Whether to clear the screen when a rebuild is triggered.
 
+#### watch.skipWrite
+Type: `boolean`<br>
+<!-- CLI: `--watch.skipWrite`<br> -->
+Default: `false`
+
+Whether to skip the `bundle.write()` step when a rebuild is triggered.
+
 #### watch.exclude
 Type: `string`<br>
 CLI: `--watch.exclude <excludedPattern>`

src/rollup/types.d.ts

@@ -617,6 +617,7 @@ export interface WatcherOptions {
 	clearScreen?: boolean;
 	exclude?: string[];
 	include?: string[];
+	skipWrite?: boolean;
 }
 
 export interface RollupWatchOptions extends InputOptions {

src/watch/watch.ts

@@ -109,6 +109,7 @@ export class Task {
 	private invalidated = true;
 	private outputFiles: string[];
 	private outputs: OutputOptions[];
+	private skipWrite: boolean;
 	private watched: Set<string>;
 	private watcher: Watcher;
 
@@ -117,6 +118,7 @@ export class Task {
 		this.closed = false;
 		this.watched = new Set();
 
+		this.skipWrite = config.watch && !!(config.watch as GenericConfigObject).skipWrite;
 		const { inputOptions, outputOptions } = mergeOptions(config);
 		this.inputOptions = inputOptions;
 		this.outputs = outputOptions;
@@ -174,7 +176,7 @@ export class Task {
 				return;
 			}
 			this.updateWatchedFiles(result);
-			await Promise.all(this.outputs.map(output => result.write(output)));
+			this.skipWrite || (await Promise.all(this.outputs.map(output => result.write(output))));
 			this.watcher.emit('event', {
 				code: 'BUNDLE_END',
 				duration: Date.now() - start,

test/watch/index.js

@@ -1009,6 +1009,71 @@ describe('rollup.watch', () => {
 		]);
 	});
 
+	it('skips filesystem writes when configured', () => {
+		let watchChangeCnt = 0;
+		return sander
+			.copydir('test/watch/samples/skip-writes')
+			.to('test/_tmp/input')
+			.then(() => {
+				watcher = rollup.watch({
+					input: 'test/_tmp/input/main.js',
+					output: {
+						file: 'test/_tmp/output/bundle.js',
+						format: 'cjs'
+					},
+					watch: {
+						skipWrite: true
+					},
+					plugins: {
+						watchChange(id) {
+							watchChangeCnt++;
+							assert.strictEqual(id, path.resolve('test/_tmp/input/main.js'));
+						}
+					}
+				});
+
+				return sequence(watcher, [
+					'START',
+					'BUNDLE_START',
+					'BUNDLE_END',
+					'END',
+					() => {
+						assert.strictEqual(sander.existsSync('../_tmp/output/bundle.js'), false);
+						assert.strictEqual(watchChangeCnt, 0);
+						sander.writeFileSync('test/_tmp/input/main.js', 'export default 43;');
+					},
+					'START',
+					'BUNDLE_START',
+					'BUNDLE_END',
+					'END',
+					() => {
+						assert.strictEqual(sander.existsSync('../_tmp/output/bundle.js'), false);
+						assert.strictEqual(watchChangeCnt, 1);
+						sander.writeFileSync('test/_tmp/input/main.js', 'export default 43;');
+					},
+					'START',
+					'BUNDLE_START',
+					'BUNDLE_END',
+					'END',
+					() => {
+						assert.strictEqual(sander.existsSync('../_tmp/output/bundle.js'), false);
+						assert.strictEqual(watchChangeCnt, 2);
+						sander.writeFileSync('test/_tmp/input/main.js', 'export default 43;');
+					},
+					'START',
+					'BUNDLE_START',
+					'BUNDLE_END',
+					// 'END',
+					evt => {
+						assert.strictEqual(sander.existsSync('../_tmp/output/bundle.js'), false);
+						assert.strictEqual(watchChangeCnt, 3);
+						// still aware of its output destination
+						assert.strictEqual(evt.output[0], path.resolve('test/_tmp/output/bundle.js'));
+					}
+				]);
+			});
+	});
+
 	describe('addWatchFile', () => {
 		it('supports adding additional watch files in plugin hooks', () => {
 			const watchChangeIds = [];

test/watch/samples/skip-writes/main.js

@@ -0,0 +1 @@
+export default 42;