require implementations: Make module map an array, preset length, if number of modules is known

Summary: For specific cases, Metro will write the number of modules to the bundle. The require implementation can take advantage of that, and construct an array with the target size.

Reviewed By: mjesun

Differential Revision: D7696290

fbshipit-source-id: a7be74c02960dc089e4d3c1accd7c732b762c8b5

packages/metro/src/ModuleGraph/build.js

@@ -52,7 +52,12 @@ async function build(options: BuildOptions): Promise<BuildResult> {
   ]);
 
   const {entryModules} = graph;
-  const preludeScript = virtualModule(getPreludeCode({isDev: !optimize}));
+  const preludeScript = virtualModule(
+    getPreludeCode({
+      extraVars: {__NUM_MODULES__: graph.modules.length},
+      isDev: !optimize,
+    }),
+  );
   const prependedScripts = [preludeScript, ...moduleSystem, ...polyfills];
   return {
     entryModules,

packages/metro/src/ModuleGraph/types.flow.js

@@ -98,9 +98,9 @@ export type Module = {|
 |};
 
 export type PostProcessModules = (
-  modules: Iterable<Module>,
+  modules: $ReadOnlyArray<Module>,
   entryPoints: Array<string>,
-) => Iterable<Module>;
+) => $ReadOnlyArray<Module>;
 
 export type OutputFn<
   M: FBSourceMap | MetroSourceMap = FBSourceMap | MetroSourceMap,

packages/metro/src/integration_tests/__tests__/__snapshots__/basic_bundle-test.js.snap

@@ -1,17 +1,17 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`basic_bundle bundles package with polyfills 1`] = `
-"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='production';
+"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV=\\"production\\";
 (function (global) {
   'use strict';
 
   var PRINT_REQUIRE_PATHS = false;
   global.require = metroRequire;
   global.__d = define;
-  var modules = Object.create(null);
+  var modules = typeof __NUM_MODULES__ === 'number' ? Array(__NUM_MODULES__ | 0) : Object.create(null);
 
   function define(factory, moduleId, dependencyMap) {
-    if (moduleId in modules) {
+    if (modules[moduleId] != null) {
       return;
     }
 
@@ -204,17 +204,17 @@ require(0);"
 `;
 
 exports[`basic_bundle bundles package without polyfills 1`] = `
-"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV='production';
+"var __DEV__=false,__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),process=this.process||{};process.env=process.env||{};process.env.NODE_ENV=\\"production\\";
 (function (global) {
   'use strict';
 
   var PRINT_REQUIRE_PATHS = false;
   global.require = metroRequire;
   global.__d = define;
-  var modules = Object.create(null);
+  var modules = typeof __NUM_MODULES__ === 'number' ? Array(__NUM_MODULES__ | 0) : Object.create(null);
 
   function define(factory, moduleId, dependencyMap) {
-    if (moduleId in modules) {
+    if (modules[moduleId] != null) {
       return;
     }
 

packages/metro/src/lib/__tests__/getPreludeCode-test.js

@@ -16,22 +16,44 @@ const vm = require('vm');
 
 ['development', 'production'].forEach(mode => {
   describe(`${mode} mode`, () => {
+    const isDev = mode === 'development';
+
     it('sets up `process.env.NODE_ENV` and `__DEV__`', () => {
       const sandbox: $FlowFixMe = {};
       vm.createContext(sandbox);
-      vm.runInContext(getPreludeCode({isDev: mode == 'development'}), sandbox);
+      vm.runInContext(getPreludeCode({isDev}), sandbox);
       expect(sandbox.process.env.NODE_ENV).toEqual(mode);
-      expect(sandbox.__DEV__).toEqual(mode == 'development');
+      expect(sandbox.__DEV__).toEqual(isDev);
     });
 
     it('does not override an existing `process.env`', () => {
       const nextTick = () => {};
       const sandbox: $FlowFixMe = {process: {nextTick, env: {FOOBAR: 123}}};
       vm.createContext(sandbox);
-      vm.runInContext(getPreludeCode({isDev: mode == 'development'}), sandbox);
+      vm.runInContext(getPreludeCode({isDev}), sandbox);
       expect(sandbox.process.env.NODE_ENV).toEqual(mode);
       expect(sandbox.process.env.FOOBAR).toEqual(123);
       expect(sandbox.process.nextTick).toEqual(nextTick);
     });
+
+    it('allows to define additional variables', () => {
+      const sandbox: $FlowFixMe = {};
+      const FOO = '1';
+      const BAR = 2;
+      vm.createContext(sandbox);
+      vm.runInContext(getPreludeCode({isDev, extraVars: {FOO, BAR}}), sandbox);
+      expect(sandbox.FOO).toBe(FOO);
+      expect(sandbox.BAR).toBe(BAR);
+    });
+
+    it('does not override core variables with additional variables', () => {
+      const sandbox: $FlowFixMe = {};
+      vm.createContext(sandbox);
+      vm.runInContext(
+        getPreludeCode({isDev, extraVars: {__DEV__: 123}}),
+        sandbox,
+      );
+      expect(sandbox.__DEV__).toBe(isDev);
+    });
   });
 });

packages/metro/src/lib/getPreludeCode.js

@@ -10,13 +10,36 @@
 
 'use strict';
 
-function getPreludeCode({isDev}: {|+isDev: boolean|}): string {
-  return (
-    `var __DEV__=${String(isDev)},` +
-    '__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now(),' +
-    'process=this.process||{};process.env=process.env||{};' +
-    `process.env.NODE_ENV='${isDev ? 'development' : 'production'}';`
-  );
+function getPreludeCode({
+  extraVars,
+  isDev,
+}: {|
+  +extraVars?: {[string]: mixed},
+  +isDev: boolean,
+|}): string {
+  const vars = [
+    ...formatExtraVars(extraVars),
+    `__DEV__=${String(isDev)}`,
+    '__BUNDLE_START_TIME__=this.nativePerformanceNow?nativePerformanceNow():Date.now()',
+    'process=this.process||{}',
+  ];
+  return `var ${vars.join(',')};${processEnv(
+    isDev ? 'development' : 'production',
+  )}`;
+}
+
+function formatExtraVars(extraVars) {
+  let assignments = [];
+  for (const key in extraVars) {
+    assignments.push(`${key}=${JSON.stringify(extraVars[key])}`);
+  }
+  return assignments;
+}
+
+function processEnv(nodeEnv) {
+  return `process.env=process.env||{};process.env.NODE_ENV=${JSON.stringify(
+    nodeEnv,
+  )};`;
 }
 
 module.exports = getPreludeCode;

packages/metro/src/lib/polyfills/require.js

@@ -14,6 +14,7 @@
 /* eslint-disable no-bitwise */
 
 declare var __DEV__: boolean;
+declare var __NUM_MODULES__: mixed;
 
 type DependencyMap = Array<ModuleID>;
 type Exports = any;
@@ -47,7 +48,6 @@ type ModuleDefinition = {|
   verboseName?: string,
   path?: string,
 |};
-type ModuleMap = {[key: ModuleID]: ModuleDefinition, __proto__: null};
 type PatchedModules = {[ModuleID]: boolean};
 type RequireFn = (id: ModuleID | VerboseModuleNameForDev) => Exports;
 type VerboseModuleNameForDev = string;
@@ -60,7 +60,10 @@ const PRINT_REQUIRE_PATHS = false;
 global.require = metroRequire;
 global.__d = define;
 
-const modules: ModuleMap = Object.create(null);
+const modules =
+  typeof __NUM_MODULES__ === 'number'
+    ? (Array(__NUM_MODULES__ | 0): Array<ModuleDefinition>)
+    : (Object.create(null): {[number]: ModuleDefinition, __proto__: null});
 if (__DEV__) {
   var verboseNamesToModuleIds: {
     [key: string]: number,
@@ -73,7 +76,7 @@ function define(
   moduleId: number,
   dependencyMap?: DependencyMap,
 ) {
-  if (moduleId in modules) {
+  if (modules[moduleId] != null) {
     if (__DEV__) {
       // (We take `inverseDependencies` from `arguments` to avoid an unused
       // named parameter in `define` in production.