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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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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.PolyfillFindingCallback.Polyfill;
import com.google.javascript.jscomp.PolyfillFindingCallback.PolyfillUsage;
import com.google.javascript.jscomp.PolyfillFindingCallback.Polyfills;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.resources.ResourceLoader;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.LinkedHashSet;
import java.util.Set;

/**
 * Injects polyfill libraries to ensure that ES6 library functions are available.
 */
public class RewritePolyfills implements HotSwapCompilerPass {

  static final DiagnosticType INSUFFICIENT_OUTPUT_VERSION_ERROR = DiagnosticType.disabled(
      "JSC_INSUFFICIENT_OUTPUT_VERSION",
      "Built-in ''{0}'' not supported in output version {1}");

  private final AbstractCompiler compiler;
  private final Polyfills polyfills;
  private final boolean isolatePolyfills;
  private Set libraries;

  public RewritePolyfills(AbstractCompiler compiler, boolean isolatePolyfills) {
    this(
        compiler,
        Polyfills.fromTable(
            ResourceLoader.loadTextResource(RewritePolyfills.class, "js/polyfills.txt")),
        isolatePolyfills);
  }

  @VisibleForTesting
  RewritePolyfills(AbstractCompiler compiler, Polyfills polyfills, boolean isolatePolyfills) {
    this.compiler = compiler;
    this.polyfills = polyfills;
    this.isolatePolyfills = isolatePolyfills;
  }

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    this.libraries = new LinkedHashSet<>();
    new PolyfillFindingCallback(compiler, polyfills).traverse(scriptRoot, this::inject);

    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"));
      compiler
          .getSynthesizedExternsInputAtEnd()
          .getAstRoot(compiler)
          .addChildToBack(jscompLookupMethodDecl);
    }

    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 $jscomp.polyfill calls whose 3rd parameter (the language version
  // that already contains the library) is the same or lower than languageOut.
  private void removeUnneededPolyfills(Node parent, Node runtimeEnd) {
    Node node = parent.getFirstChild();
    while (node != null && node != runtimeEnd) {
      Node next = node.getNext();
      if (NodeUtil.isExprCall(node)) {
        Node call = node.getFirstChild();
        Node name = call.getFirstChild();
        if (name.matchesQualifiedName("$jscomp.polyfill")) {
          FeatureSet nativeVersion =
              FeatureSet.valueOf(name.getNext().getNext().getNext().getString());
          if (languageOutIsAtLeast(nativeVersion)) {
            NodeUtil.removeChild(parent, node);
          }
        }
      }
      node = next;
    }
  }

  @Override
  public void process(Node externs, Node root) {
    hotSwapScript(root, null);
  }

  private void inject(PolyfillUsage polyfillUsage) {
    Polyfill polyfill = polyfillUsage.polyfill();
    if (polyfill.kind.equals(Polyfill.Kind.STATIC)
        && !languageOutIsAtLeast(polyfill.polyfillVersion)) {
      compiler.report(
          JSError.make(
              polyfillUsage.node(),
              INSUFFICIENT_OUTPUT_VERSION_ERROR,
              polyfillUsage.name(),
              compiler.getOptions().getOutputFeatureSet().version()));
    }

    if (!languageOutIsAtLeast(polyfill.nativeVersion) && !polyfill.library.isEmpty()) {
      libraries.add(polyfill.library);
    }
  }

  private boolean languageOutIsAtLeast(FeatureSet featureSet) {
    return PolyfillFindingCallback.languageOutIsAtLeast(
        featureSet, compiler.getOptions().getOutputFeatureSet());
  }
}