Allow OnResolve plugins to mark modules as side effect free

CHANGELOG.md

@@ -57,6 +57,36 @@
 
     This fix was contributed by [@lbwa](https://github.com/lbwa).
 
+* Plugins can now specify `sideEffects: false` ([#1009](https://github.com/evanw/esbuild/issues/1009))
+
+    The default path resolution behavior in esbuild determines if a given file can be considered side-effect free (in the [Webpack-specific sense](https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free)) by reading the contents of the nearest enclosing `package.json` file and looking for `"sideEffects": false`. However, up until now this was impossible to achieve in an esbuild plugin because there was no way of returning this metadata back to esbuild.
+
+    With this release, esbuild plugins can now return `sideEffects: false` to mark a file as having no side effects. Here's an example:
+
+    ```js
+    esbuild.build({
+      entryPoints: ['app.js'],
+      bundle: true,
+      plugins: [{
+        name: 'env-plugin',
+        setup(build) {
+          build.onResolve({ filter: /^env$/ }, args => ({
+            path: args.path,
+            namespace: 'some-ns',
+            sideEffects: false,
+          }))
+          build.onLoad({ filter: /.*/, namespace: 'some-ns' }, () => ({
+            contents: `export default self.env || (self.env = getEnv())`,
+          }))
+        },
+      }],
+    })
+    ```
+
+    This plugin creates a virtual module that can be generated by importing the string `env`. However, since the plugin returns `sideEffects: false`, the generated virtual module will not be included in the bundle if all of the imported values from the module `env` end up being unused.
+
+    This feature was contributed by [@chriscasola](https://github.com/chriscasola).
+
 ## 0.12.6
 
 * Improve template literal lowering transformation conformance ([#1327](https://github.com/evanw/esbuild/issues/1327))

cmd/esbuild/service.go

@@ -715,6 +715,13 @@ func (service *serviceType) convertPlugins(key int, jsPlugins interface{}) ([]ap
 				if value, ok := response["external"]; ok {
 					result.External = value.(bool)
 				}
+				if value, ok := response["sideEffects"]; ok {
+					if value.(bool) {
+						result.SideEffects = api.SideEffectsTrue
+					} else {
+						result.SideEffects = api.SideEffectsFalse
+					}
+				}
 				if value, ok := response["pluginData"]; ok {
 					result.PluginData = value.(int)
 				}

internal/bundler/bundler.go

@@ -738,10 +738,18 @@ func runOnResolvePlugins(
 				return nil, true, resolver.DebugMeta{}
 			}
 
+			var sideEffectsData *resolver.SideEffectsData
+			if result.IsSideEffectFree {
+				sideEffectsData = &resolver.SideEffectsData{
+					PluginName: pluginName,
+				}
+			}
+
 			return &resolver.ResolveResult{
-				PathPair:   resolver.PathPair{Primary: result.Path},
-				IsExternal: result.External,
-				PluginData: result.PluginData,
+				PathPair:               resolver.PathPair{Primary: result.Path},
+				IsExternal:             result.External,
+				PluginData:             result.PluginData,
+				PrimarySideEffectsData: sideEffectsData,
 			}, false, resolver.DebugMeta{}
 		}
 	}
@@ -1701,19 +1709,24 @@ func (s *scanner) processScannedFiles() []scannerFile {
 						// effect.
 						otherModule.SideEffects.Kind != graph.NoSideEffects_PureData_FromPlugin {
 						var notes []logger.MsgData
+						var by string
 						if data := otherModule.SideEffects.Data; data != nil {
-							var text string
-							if data.IsSideEffectsArrayInJSON {
-								text = "It was excluded from the \"sideEffects\" array in the enclosing \"package.json\" file"
+							if data.PluginName != "" {
+								by = fmt.Sprintf(" by plugin %q", data.PluginName)
 							} else {
-								text = "\"sideEffects\" is false in the enclosing \"package.json\" file"
+								var text string
+								if data.IsSideEffectsArrayInJSON {
+									text = "It was excluded from the \"sideEffects\" array in the enclosing \"package.json\" file"
+								} else {
+									text = "\"sideEffects\" is false in the enclosing \"package.json\" file"
+								}
+								tracker := logger.MakeLineColumnTracker(data.Source)
+								notes = append(notes, logger.RangeData(&tracker, data.Range, text))
 							}
-							tracker := logger.MakeLineColumnTracker(data.Source)
-							notes = append(notes, logger.RangeData(&tracker, data.Range, text))
 						}
 						s.log.AddRangeWarningWithNotes(&tracker, record.Range,
-							fmt.Sprintf("Ignoring this import because %q was marked as having no side effects",
-								otherModule.Source.PrettyPath), notes)
+							fmt.Sprintf("Ignoring this import because %q was marked as having no side effects%s",
+								otherModule.Source.PrettyPath, by), notes)
 					}
 				}
 			}

internal/config/config.go

@@ -497,9 +497,10 @@ type OnResolveArgs struct {
 type OnResolveResult struct {
 	PluginName string
 
-	Path       logger.Path
-	External   bool
-	PluginData interface{}
+	Path             logger.Path
+	External         bool
+	IsSideEffectFree bool
+	PluginData       interface{}
 
 	Msgs        []logger.Msg
 	ThrownError error

internal/resolver/resolver.go

@@ -83,6 +83,9 @@ type SideEffectsData struct {
 	Source *logger.Source
 	Range  logger.Range
 
+	// If non-empty, this false value came from a plugin
+	PluginName string
+
 	// If true, "sideEffects" was an array. If false, "sideEffects" was false.
 	IsSideEffectsArrayInJSON bool
 }

lib/shared/common.ts

@@ -780,6 +780,7 @@ export function createChannel(streamIn: StreamIn): StreamOut {
                 let path = getFlag(result, keys, 'path', mustBeString);
                 let namespace = getFlag(result, keys, 'namespace', mustBeString);
                 let external = getFlag(result, keys, 'external', mustBeBoolean);
+                let sideEffects = getFlag(result, keys, 'sideEffects', mustBeBoolean);
                 let pluginData = getFlag(result, keys, 'pluginData', canBeAnything);
                 let errors = getFlag(result, keys, 'errors', mustBeArray);
                 let warnings = getFlag(result, keys, 'warnings', mustBeArray);
@@ -792,6 +793,7 @@ export function createChannel(streamIn: StreamIn): StreamOut {
                 if (path != null) response.path = path;
                 if (namespace != null) response.namespace = namespace;
                 if (external != null) response.external = external;
+                if (sideEffects != null) response.sideEffects = sideEffects;
                 if (pluginData != null) response.pluginData = stash.store(pluginData);
                 if (errors != null) response.errors = sanitizeMessages(errors, 'errors', stash, name);
                 if (warnings != null) response.warnings = sanitizeMessages(warnings, 'warnings', stash, name);

lib/shared/stdio_protocol.ts

@@ -157,6 +157,7 @@ export interface OnResolveResponse {
 
   path?: string;
   external?: boolean;
+  sideEffects?: boolean;
   namespace?: string;
   pluginData?: number;
 

lib/shared/types.ts

@@ -241,6 +241,7 @@ export interface OnResolveResult {
 
   path?: string;
   external?: boolean;
+  sideEffects?: boolean;
   namespace?: string;
   pluginData?: any;
 

pkg/api/api.go

@@ -427,6 +427,13 @@ func Serve(serveOptions ServeOptions, buildOptions BuildOptions) (ServeResult, e
 ////////////////////////////////////////////////////////////////////////////////
 // Plugin API
 
+type SideEffects uint8
+
+const (
+	SideEffectsTrue SideEffects = iota
+	SideEffectsFalse
+)
+
 type Plugin struct {
 	Name  string
 	Setup func(PluginBuild)
@@ -465,10 +472,11 @@ type OnResolveResult struct {
 	Errors   []Message
 	Warnings []Message
 
-	Path       string
-	External   bool
-	Namespace  string
-	PluginData interface{}
+	Path        string
+	External    bool
+	SideEffects SideEffects
+	Namespace   string
+	PluginData  interface{}
 
 	WatchFiles []string
 	WatchDirs  []string

pkg/api/api_impl.go

@@ -1470,6 +1470,7 @@ func (impl *pluginImpl) OnResolve(options OnResolveOptions, callback func(OnReso
 
 			result.Path = logger.Path{Text: response.Path, Namespace: response.Namespace}
 			result.External = response.External
+			result.IsSideEffectFree = response.SideEffects == SideEffectsFalse
 			result.PluginData = response.PluginData
 
 			// Convert log messages

scripts/plugin-tests.js

@@ -709,6 +709,80 @@ let pluginTests = {
     assert.strictEqual(result.default, 123)
   },
 
+  async resolveWithSideEffectsFalse({ esbuild, testDir }) {
+    const input = path.join(testDir, 'in.js')
+
+    await writeFileAsync(input, `
+      import './re-export-unused'
+      import {a, b, c} from './re-export-used'
+      import './import-unused'
+      use([a, b, c])
+    `)
+    await writeFileAsync(path.join(testDir, 're-export-unused.js'), `
+      export {default as a} from 'plugin:unused-false'
+      export {default as b} from 'plugin:unused-true'
+      export {default as c} from 'plugin:unused-none'
+    `)
+    await writeFileAsync(path.join(testDir, 're-export-used.js'), `
+      export {default as a} from 'plugin:used-false'
+      export {default as b} from 'plugin:used-true'
+      export {default as c} from 'plugin:used-none'
+    `)
+    await writeFileAsync(path.join(testDir, 'import-unused.js'), `
+      import 'plugin:ignored-false'
+      import 'plugin:ignored-true'
+      import 'plugin:ignored-none'
+    `)
+
+    const result = await esbuild.build({
+      entryPoints: [input],
+      bundle: true,
+      write: false,
+      format: 'cjs',
+      logLevel: 'error',
+      plugins: [{
+        name: 'name',
+        setup(build) {
+          build.onResolve({ filter: /^plugin:/ }, args => {
+            return {
+              path: args.path,
+              namespace: 'ns',
+              sideEffects:
+                args.path.endsWith('-true') ? true :
+                  args.path.endsWith('-false') ? false :
+                    undefined,
+            };
+          });
+          build.onLoad({ filter: /^plugin:/ }, args => {
+            return { contents: `export default use(${JSON.stringify(args.path)})` };
+          });
+        },
+      }],
+    })
+
+    // Validate that the unused "sideEffects: false" files were omitted
+    const used = [];
+    new Function('use', result.outputFiles[0].text)(x => used.push(x));
+    assert.deepStrictEqual(used, [
+      'plugin:unused-true',
+      'plugin:unused-none',
+
+      'plugin:used-false',
+      'plugin:used-true',
+      'plugin:used-none',
+
+      'plugin:ignored-true',
+      'plugin:ignored-none',
+
+      [3, 4, 5],
+    ])
+
+    // Check that the warning for "sideEffect: false" imports mentions the plugin
+    assert.strictEqual(result.warnings.length, 1)
+    assert.strictEqual(result.warnings[0].text,
+      'Ignoring this import because "ns:plugin:ignored-false" was marked as having no side effects by plugin "name"')
+  },
+
   async noResolveDirInFileModule({ esbuild, testDir }) {
     const input = path.join(testDir, 'in.js')
     const output = path.join(testDir, 'out.js')