implement custom conditions for "exports"

CHANGELOG.md

@@ -91,6 +91,8 @@
 
     Note that when you use conditions, _your package may end up in the bundle multiple times!_ This is a subtle issue that can cause bugs due to duplicate copies of your code's state in addition to bloating the resulting bundle. This is commonly known as the [dual package hazard](https://nodejs.org/docs/latest/api/packages.html#packages_dual_package_hazard). The primary way of avoiding this is to put all of your code in the `require` condition and have the `import` condition just be a light wrapper that calls `require` on your package and re-exports the package using ESM syntax.
 
+    There is also support for custom conditions with the `--conditions=` flag. The meaning of these is entirely up to package authors. For example, you could imagine a package that requires you to configure `--conditions=test,en-US`. Node has currently only endorsed the `development` and `production` custom conditions for recommended use.
+
 ## 0.8.57
 
 * Fix overlapping chunk names when code splitting is active ([#928](https://github.com/evanw/esbuild/issues/928))

internal/bundler/bundler_packagejson_test.go

@@ -1663,3 +1663,31 @@ Users/user/project/node_modules/pkg1/package.json: note: The module specifier ".
 `,
 	})
 }
+
+func TestPackageJsonExportsCustomConditions(t *testing.T) {
+	packagejson_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/Users/user/project/src/entry.js": `
+				import 'pkg1'
+			`,
+			"/Users/user/project/node_modules/pkg1/package.json": `
+				{
+					"exports": {
+						"custom1": "./custom1.js",
+						"custom2": "./custom2.js",
+						"default": "./default.js"
+					}
+				}
+			`,
+			"/Users/user/project/node_modules/pkg1/custom2.js": `
+				console.log('SUCCESS')
+			`,
+		},
+		entryPaths: []string{"/Users/user/project/src/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			AbsOutputFile: "/Users/user/project/out.js",
+			Conditions:    []string{"custom2"},
+		},
+	})
+}

internal/bundler/snapshots/snapshots_packagejson.txt

@@ -430,6 +430,12 @@ TestPackageJsonExportsBrowser
 // Users/user/project/node_modules/pkg/browser.js
 console.log("SUCCESS");
 
+================================================================================
+TestPackageJsonExportsCustomConditions
+---------- /Users/user/project/out.js ----------
+// Users/user/project/node_modules/pkg1/custom2.js
+console.log("SUCCESS");
+
 ================================================================================
 TestPackageJsonExportsDefaultOverImportAndRequire
 ---------- /Users/user/project/out.js ----------

internal/config/config.go

@@ -201,6 +201,7 @@ type Options struct {
 
 	ExtensionOrder  []string
 	MainFields      []string
+	Conditions      []string
 	AbsNodePaths    []string // The "NODE_PATH" variable from Node.js
 	ExternalModules ExternalModules
 

internal/resolver/resolver.go

@@ -198,6 +198,9 @@ func NewResolver(fs fs.FS, log logger.Log, caches *cache.CacheSet, options confi
 	esmConditionsDefault := map[string]bool{}
 	esmConditionsImport := map[string]bool{"import": true}
 	esmConditionsRequire := map[string]bool{"require": true}
+	for _, condition := range options.Conditions {
+		esmConditionsDefault[condition] = true
+	}
 	switch options.Platform {
 	case config.PlatformBrowser:
 		esmConditionsDefault["browser"] = true

lib/common.ts

@@ -176,6 +176,7 @@ function flagsForBuildOptions(
   let resolveExtensions = getFlag(options, keys, 'resolveExtensions', mustBeArray);
   let nodePathsInput = getFlag(options, keys, 'nodePaths', mustBeArray);
   let mainFields = getFlag(options, keys, 'mainFields', mustBeArray);
+  let conditions = getFlag(options, keys, 'conditions', mustBeArray);
   let external = getFlag(options, keys, 'external', mustBeArray);
   let loader = getFlag(options, keys, 'loader', mustBeObject);
   let outExtension = getFlag(options, keys, 'outExtension', mustBeObject);
@@ -235,6 +236,15 @@ function flagsForBuildOptions(
     }
     flags.push(`--main-fields=${values.join(',')}`);
   }
+  if (conditions) {
+    let values: string[] = [];
+    for (let value of conditions) {
+      value += '';
+      if (value.indexOf(',') >= 0) throw new Error(`Invalid condition: ${value}`);
+      values.push(value);
+    }
+    flags.push(`--conditions=${values.join(',')}`);
+  }
   if (external) for (let name of external) flags.push(`--external:${name}`);
   if (banner) {
     for (let type in banner) {

lib/types.ts

@@ -44,6 +44,7 @@ export interface BuildOptions extends CommonOptions {
   loader?: { [ext: string]: Loader };
   resolveExtensions?: string[];
   mainFields?: string[];
+  conditions?: string[];
   write?: boolean;
   tsconfig?: string;
   outExtension?: { [ext: string]: string };

pkg/api/api.go

@@ -254,6 +254,7 @@ type BuildOptions struct {
 	Format            Format
 	External          []string
 	MainFields        []string
+	Conditions        []string // For the "exports" field in "package.json"
 	Loader            map[string]Loader
 	ResolveExtensions []string
 	Tsconfig          string

pkg/api/api_impl.go

@@ -737,6 +737,7 @@ func rebuildImpl(
 		ExternalModules:       validateExternals(log, realFS, buildOpts.External),
 		TsConfigOverride:      validatePath(log, realFS, buildOpts.Tsconfig, "tsconfig path"),
 		MainFields:            buildOpts.MainFields,
+		Conditions:            append([]string{}, buildOpts.Conditions...),
 		PublicPath:            buildOpts.PublicPath,
 		KeepNames:             buildOpts.KeepNames,
 		InjectAbsPaths:        make([]string, len(buildOpts.Inject)),
@@ -749,6 +750,9 @@ func rebuildImpl(
 		WatchMode:             buildOpts.Watch != nil,
 		Plugins:               plugins,
 	}
+	if options.MainFields != nil {
+		options.MainFields = append([]string{}, options.MainFields...)
+	}
 	for i, path := range buildOpts.Inject {
 		options.InjectAbsPaths[i] = validatePath(log, realFS, path, "inject path")
 	}

pkg/cli/cli_impl.go

@@ -180,6 +180,9 @@ func parseOptionsImpl(osArgs []string, buildOpts *api.BuildOptions, transformOpt
 		case strings.HasPrefix(arg, "--main-fields=") && buildOpts != nil:
 			buildOpts.MainFields = strings.Split(arg[len("--main-fields="):], ",")
 
+		case strings.HasPrefix(arg, "--conditions=") && buildOpts != nil:
+			buildOpts.Conditions = strings.Split(arg[len("--conditions="):], ",")
+
 		case strings.HasPrefix(arg, "--public-path=") && buildOpts != nil:
 			buildOpts.PublicPath = arg[len("--public-path="):]