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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2015 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.javascript.jscomp;

import com.google.common.annotations.VisibleForTesting;
import com.google.javascript.jscomp.PolyfillUsageFinder.Polyfill;
import com.google.javascript.jscomp.PolyfillUsageFinder.PolyfillUsage;
import com.google.javascript.jscomp.PolyfillUsageFinder.Polyfills;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.jscomp.resources.ResourceLoader;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.QualifiedName;
import java.util.LinkedHashSet;
import java.util.Set;
import org.jspecify.nullness.Nullable;

/**
 * Injects polyfill libraries to ensure that ES6+ library functions are available.
 *
 * 

Also runs if polyfill isolation is enabled, even if polyfill injection is disabled, in order * to prevent deletion of a required library function by dead code elimination. * *

TODO(b/120486392): consider merging this pass with {@link InjectRuntimeLibraries} and {@link * InjectTranspilationRuntimeLibraries}. */ public class RewritePolyfills implements CompilerPass { static final DiagnosticType INSUFFICIENT_OUTPUT_VERSION_ERROR = DiagnosticType.disabled( "JSC_INSUFFICIENT_OUTPUT_VERSION", "Built-in ''{0}'' not supported in output version {1}"); private static final QualifiedName JSCOMP_POLYFILL = QualifiedName.of("$jscomp.polyfill"); private final AbstractCompiler compiler; private final Polyfills polyfills; private final boolean injectPolyfills; private final boolean isolatePolyfills; private Set libraries; /** * @param injectPolyfills if true, injects $jscomp.polyfill initializations into the first input. * if false, no polyfills are injected. * @param isolatePolyfills if true, adds externs for library functions used by {@link * IsolatePolyfills} to prevent their deletion. */ public RewritePolyfills( AbstractCompiler compiler, boolean injectPolyfills, boolean isolatePolyfills) { this( compiler, Polyfills.fromTable( ResourceLoader.loadTextResource(RewritePolyfills.class, "js/polyfills.txt")), injectPolyfills, isolatePolyfills); } @VisibleForTesting RewritePolyfills( AbstractCompiler compiler, Polyfills polyfills, boolean injectPolyfills, boolean isolatePolyfills) { this.compiler = compiler; this.polyfills = polyfills; this.injectPolyfills = injectPolyfills; this.isolatePolyfills = isolatePolyfills; } @Override public void process(Node externs, Node root) { if (this.isolatePolyfills) { // Polyfill isolation requires a pass to run near the end of optimizations. That pass may call // into a library method injected in this pass. Adding an externs declaration of that library // method prevents it from being dead-code-elimiated before polyfill isolation runs. Node jscompLookupMethodDecl = IR.var(IR.name("$jscomp$lookupPolyfilledValue")); final Node synthesizedExternsAstRoot = compiler.getSynthesizedExternsInput().getAstRoot(compiler); jscompLookupMethodDecl.srcrefTree(synthesizedExternsAstRoot); synthesizedExternsAstRoot.addChildToBack(jscompLookupMethodDecl); compiler.reportChangeToEnclosingScope(jscompLookupMethodDecl); } if (!this.injectPolyfills) { // Nothing left to do. Probably this pass only needed to run because --isolate_polyfills is // enabled but not --rewrite_polyfills. return; } this.libraries = new LinkedHashSet<>(); new PolyfillUsageFinder(compiler, polyfills).traverseExcludingGuarded(root, this::inject); if (libraries.isEmpty()) { return; } Node lastNode = null; for (String library : libraries) { lastNode = compiler.ensureLibraryInjected(library, false); } if (lastNode != null) { Node parent = lastNode.getParent(); removeUnneededPolyfills(parent, lastNode.getNext()); compiler.reportChangeToEnclosingScope(parent); } } // Remove any $jscomp.polyfill calls whose 3rd parameter (the language version // that already contains the library) is the same or lower than languageOut. private void removeUnneededPolyfills(Node parent, Node runtimeEnd) { Node node = parent.getFirstChild(); // The target environment is assumed to support the features in this set. final FeatureSet outputFeatureSet = compiler.getOptions().getOutputFeatureSet(); while (node != null && node != runtimeEnd) { // look up the next node now, because we may be removing this one. Node next = node.getNext(); FeatureSet polyfillSupportedFeatureSet = getPolyfillSupportedFeatureSet(node); if (polyfillSupportedFeatureSet != null && outputFeatureSet.contains(polyfillSupportedFeatureSet)) { NodeUtil.removeChild(parent, node); NodeUtil.markFunctionsDeleted(node, compiler); } node = next; } } /** * If the given `Node` is a polyfill definition, return the `FeatureSet` which should be * considered to already include that polyfill (making it unnecessary). * *

Otherwise, return `null`. */ private @Nullable FeatureSet getPolyfillSupportedFeatureSet(Node maybePolyfill) { FeatureSet polyfillSupportFeatureSet = null; if (NodeUtil.isExprCall(maybePolyfill)) { Node call = maybePolyfill.getFirstChild(); Node name = call.getFirstChild(); if (JSCOMP_POLYFILL.matches(name)) { final String nativeVersionStr = name.getNext().getNext().getNext().getString(); polyfillSupportFeatureSet = FeatureSet.valueOf(nativeVersionStr); // Safari has been really slow to implement these regex features, even though it has // kept on top of the features we polyfill, so we want to ignore the regex features // when deciding whether the polyfill should be considered "already supported" in the // target environment. // NOTE: This special case seems reasonable for now, but if further divergence occurs // we should consider doing a more direct solution by having the polyfill definitions // report names of `FeatureSet` values representing browser `FeatureSet` year instead of // spec release year. polyfillSupportFeatureSet = polyfillSupportFeatureSet.without( Feature.REGEXP_FLAG_S, Feature.REGEXP_LOOKBEHIND, Feature.REGEXP_NAMED_GROUPS, Feature.REGEXP_UNICODE_PROPERTY_ESCAPE); } } return polyfillSupportFeatureSet; } private void inject(PolyfillUsage polyfillUsage) { Polyfill polyfill = polyfillUsage.polyfill(); final FeatureSet outputFeatureSet = compiler.getOptions().getOutputFeatureSet(); final FeatureSet featuresRequiredByPolyfill = FeatureSet.valueOf(polyfill.polyfillVersion); if (polyfill.kind.equals(Polyfill.Kind.STATIC) && !outputFeatureSet.contains(featuresRequiredByPolyfill)) { compiler.report( JSError.make( polyfillUsage.node(), INSUFFICIENT_OUTPUT_VERSION_ERROR, polyfillUsage.name(), outputFeatureSet.version())); } // The question we want to ask here is: // "Does the target platform already have the symbol this polyfill provides?" // We approximate it by asking instead: // "Does the target platform support all of the features that existed in the language // version that introduced this symbol?" if (!outputFeatureSet.contains(FeatureSet.valueOf(polyfill.nativeVersion)) && !polyfill.library.isEmpty()) { libraries.add(polyfill.library); } } }





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