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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 2017 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 static com.google.common.base.Preconditions.checkState;
import static com.google.javascript.jscomp.AstFactory.type;

import com.google.javascript.jscomp.colors.StandardColors;
import com.google.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.StaticScope;
import com.google.javascript.rhino.Token;

/** Converts ES6 "for of" loops to ES5. */
public final class Es6ForOfConverter extends NodeTraversal.AbstractPostOrderCallback
    implements CompilerPass {
  private final AbstractCompiler compiler;
  private static final FeatureSet transpiledFeatures = FeatureSet.BARE_MINIMUM.with(Feature.FOR_OF);
  private final DefaultNameGenerator namer;
  private final AstFactory astFactory;
  private final StaticScope namespace;

  private static final String ITER_BASE = "$jscomp$iter$";

  private static final String ITER_RESULT = "$jscomp$key$";

  public Es6ForOfConverter(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.namer = new DefaultNameGenerator();
    this.astFactory = compiler.createAstFactory();
    this.namespace = compiler.getTranspilationNamespace();
  }

  @Override
  public void process(Node externs, Node root) {
    TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this);
    TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, root, transpiledFeatures);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isForOf()) {
      visitForOf(n);
    }
  }

  // TODO(lharker): break up this method
  private void visitForOf(Node node) {
    // `var v` or `let v` or v any valid lhs
    Node variable = node.removeFirstChild();
    Node iterable = node.removeFirstChild();
    Node body = node.removeFirstChild();
    JSDocInfo varJSDocInfo = variable.getJSDocInfo();
    // `$jscomp$iter$0`
    Node iterName =
        astFactory.createName(
            ITER_BASE + compiler.getUniqueNameIdSupplier().get(), type(StandardColors.ITERATOR_ID));
    iterName.makeNonIndexable();
    // `$jscomp$iter$0.next()`
    Node getNext =
        astFactory.createCallWithUnknownType(
            astFactory.createGetPropWithUnknownType(iterName.cloneTree(), "next"));
    // generate a unique iterator result name for every for-of loop getting rewritten to avoid
    // conflicts
    String iteratorResultName =
        ITER_RESULT
            + compiler
                .getUniqueIdSupplier()
                .getUniqueId(compiler.getInput(NodeUtil.getEnclosingScript(node).getInputId()))
            + "$";

    if (NodeUtil.isNameDeclaration(variable)) {
      iteratorResultName += variable.getFirstChild().getString();
    } else if (variable.isName()) {
      iteratorResultName += variable.getString();
    } else {
      // give arbitrary lhs expressions an arbitrary name
      iteratorResultName += namer.generateNextName();
    }
    // `$jscomp$key$extraName`
    Node iterResult = astFactory.createNameWithUnknownType(iteratorResultName);
    iterResult.makeNonIndexable();
    // `$jscomp.makeIterator(iterable)`
    Node callMakeIterator =
        astFactory
            .createJSCompMakeIteratorCall(iterable, this.namespace)
            .srcrefTreeIfMissing(iterable);
    // `var $jscomp$iter$0 = $jscomp.makeIterator(iterable)`
    Node initIter = IR.var(iterName, callMakeIterator).srcrefTreeIfMissing(iterable);
    // var $jscomp$key$extraName = $jscomp$iter$0.next();
    Node initIterResult =
        IR.var(iterResult.cloneTree(), getNext.cloneTree()).srcrefTreeIfMissing(iterable);
    // !$jscomp$key$extraName.done
    Node cond =
        astFactory.createNot(
            astFactory.createGetProp(iterResult.cloneTree(), "done", type(StandardColors.BOOLEAN)));
    // $jscomp$key$extraName = $jscomp$iter$0.next()
    Node incr = astFactory.createAssign(iterResult.cloneTree(), getNext.cloneTree());

    Node declarationOrAssign;
    if (!NodeUtil.isNameDeclaration(variable)) {
      // e.g. `for(a.b of []) {}`
      declarationOrAssign =
          astFactory.createAssign(
              variable.cloneTree().setJSDocInfo(null),
              astFactory.createGetProp(iterResult.cloneTree(), "value", type(variable)));
      declarationOrAssign.setJSDocInfo(varJSDocInfo);
      declarationOrAssign = IR.exprResult(declarationOrAssign);
    } else {
      // `for(let a of []) {}` or `for(const a of []) {}`
      AstFactory.Type type = type(variable.getFirstChild());
      Token declarationType = variable.getToken(); // i.e. VAR, CONST, or LET.
      checkState(
          !declarationType.equals(Token.VAR),
          "var initializers must've gotten moved out of the loop during normalize");
      declarationOrAssign =
          astFactory.createSingleNameDeclaration(
              declarationType,
              variable.getFirstChild().getString(),
              astFactory.createGetProp(iterResult.cloneTree(), "value", type));
      declarationOrAssign.setJSDocInfo(varJSDocInfo);
    }
    Node newBody = IR.block(declarationOrAssign, body).srcref(body);
    Node empty = astFactory.createEmpty();
    Node newFor = IR.forNode(empty, cond, incr, newBody).srcrefTreeIfMissing(node);
    node.replaceWith(newFor);
    // Check if the for loop has a parent that is a label i.e. `loop 1: for(...of .. .)`
    Node insertionPoint = newFor;
    while (insertionPoint.getParent().isLabel()) {
      insertionPoint = insertionPoint.getParent();
    }
    initIter.insertBefore(insertionPoint);
    initIterResult.insertAfter(initIter);
    compiler.reportChangeToEnclosingScope(newFor);
  }
}