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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 2014 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.javascript.jscomp.AstFactory.type;

import com.google.common.collect.Lists;
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.Node;
import com.google.javascript.rhino.QualifiedName;
import com.google.javascript.rhino.Token;
import java.util.ArrayList;
import java.util.List;
import org.jspecify.nullness.Nullable;

/**
 * Converts ES6 code to valid ES5 code. This class does most of the transpilation, and
 * https://github.com/google/closure-compiler/wiki/ECMAScript6 lists which ES6 features are
 * supported. Other classes that start with "Es6" do other parts of the transpilation.
 *
 * 

In most cases, the output is valid as ES3 (hence the class name) but in some cases, if the * output language is set to ES5, we rely on ES5 features such as getters, setters, and * Object.defineProperties. */ // TODO(tbreisacher): This class does too many things. Break it into smaller passes. public final class LateEs6ToEs3Converter implements NodeTraversal.Callback, CompilerPass { private final AbstractCompiler compiler; private final AstFactory astFactory; private final Es6TemplateLiterals templateLiteralConverter; private static final FeatureSet transpiledFeatures = FeatureSet.BARE_MINIMUM.with( Feature.COMPUTED_PROPERTIES, Feature.MEMBER_DECLARATIONS, Feature.TEMPLATE_LITERALS); // We want to insert the call to `var tagFnFirstArg = $jscomp.createTemplateTagFirstArg...` just // before this node. For the first script, this node is right after the runtime injected function // definition as injecting to the top of the script causes runtime errors // https://github.com/google/closure-compiler/issues/3589. For the subsequent script(s), the call // is injected to the top of that script. private @Nullable Node templateLitInsertionPoint = null; private static final String FRESH_COMP_PROP_VAR = "$jscomp$compprop"; public LateEs6ToEs3Converter(AbstractCompiler compiler) { this.compiler = compiler; this.astFactory = compiler.createAstFactory(); this.templateLiteralConverter = new Es6TemplateLiterals(compiler); } @Override public void process(Node externs, Node root) { TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this); TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, root, transpiledFeatures); } @Override public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) { switch (n.getToken()) { case SCRIPT: // For all scripts, initialize the injection point to be top of the script templateLitInsertionPoint = n.getFirstChild(); break; case GETTER_DEF: case SETTER_DEF: if (FeatureSet.ES3.contains(compiler.getOptions().getOutputFeatureSet())) { TranspilationUtil.cannotConvert( compiler, n, "ES5 getters/setters (consider using --language_out=ES5)"); return false; } break; default: break; } return true; } @Override public void visit(NodeTraversal t, Node n, Node parent) { switch (n.getToken()) { case ASSIGN: // Find whether this script contains the `$jscomp.createTemplateTagFirstArgWithRaw = // function(..) {..}` node. If yes, update the templateLitInsertionPoint. Node lhs = n.getFirstChild(); Node rhs = n.getSecondChild(); if (lhs.isGetProp() && rhs.isFunction() && lhs.getFirstChild().isName()) { QualifiedName qName = QualifiedName.of("$jscomp.createTemplateTagFirstArgWithRaw"); if (qName.matches(lhs)) { checkNotNull(n.getParent(), n); checkState(n.getParent().isExprResult(), n); templateLitInsertionPoint = n.getParent().getNext(); } } break; case OBJECTLIT: visitObject(n); break; case MEMBER_FUNCTION_DEF: if (parent.isObjectLit()) { visitMemberFunctionDefInObjectLit(n); } break; case TAGGED_TEMPLATELIT: templateLiteralConverter.visitTaggedTemplateLiteral(t, n, templateLitInsertionPoint); break; case TEMPLATELIT: if (!parent.isTaggedTemplateLit()) { templateLiteralConverter.visitTemplateLiteral(t, n); } break; default: break; } } /** * Converts a member definition in an object literal to an ES3 key/value pair. Member definitions * in classes are handled in {@link Es6RewriteClass}. */ private void visitMemberFunctionDefInObjectLit(Node n) { String name = n.getString(); Node nameNode = n.getFirstFirstChild(); Node stringKey = astFactory.createStringKey(name, n.removeFirstChild()); stringKey.setJSDocInfo(n.getJSDocInfo()); n.replaceWith(stringKey); stringKey.srcref(nameNode); compiler.reportChangeToEnclosingScope(stringKey); } private void visitObject(Node obj) { for (Node child = obj.getFirstChild(); child != null; child = child.getNext()) { if (child.isComputedProp()) { visitObjectWithComputedProperty(obj); return; } } } /** * Transpiles an object node with computed property, * and add type information to the new nodes if this pass ran after type checking. * For example,

   {@code
   *   var obj = {a: 1, [i++]: 2}
   *   is transpiled to
   *   var $jscomp$compprop0 = {};
   *   var obj = ($jscomp$compprop0.a = 1, ($jscomp$compprop0[i++] = 2, $jscomp$compprop0));
   * }
* Note that when adding type information to the nodes, the NAME node $jscomp$compprop0 * would always be assigned the type of the entire object (in the above example {a: number}). * This is because we do not have sufficient type information during transpilation to know, * for example, $jscomp$compprop0 has type Object{} in the expression $jscomp$compprop0.a = 1 */ private void visitObjectWithComputedProperty(Node obj) { checkArgument(obj.isObjectLit()); List props = new ArrayList<>(); Node currElement = obj.getFirstChild(); AstFactory.Type objectType = type(obj); while (currElement != null) { if (currElement.getBooleanProp(Node.COMPUTED_PROP_GETTER) || currElement.getBooleanProp(Node.COMPUTED_PROP_SETTER)) { TranspilationUtil.cannotConvertYet( compiler, currElement, "computed getter/setter in an object literal"); return; } else if (currElement.isGetterDef() || currElement.isSetterDef()) { currElement = currElement.getNext(); } else { Node nextNode = currElement.getNext(); currElement.detach(); props.add(currElement); currElement = nextNode; } } String objName = FRESH_COMP_PROP_VAR + compiler.getUniqueNameIdSupplier().get(); props = Lists.reverse(props); Node result = astFactory.createName(objName, objectType); for (Node propdef : props) { if (propdef.isComputedProp()) { Node propertyExpression = propdef.removeFirstChild(); Node value = propdef.removeFirstChild(); result = astFactory.createComma( astFactory.createAssign( astFactory.createGetElem( astFactory.createName(objName, objectType), propertyExpression), value), result); } else { Node val = propdef.removeFirstChild(); boolean isQuotedAccess = propdef.isQuotedStringKey(); propdef.setToken(Token.STRINGLIT); propdef.setColor(StandardColors.STRING); propdef.putBooleanProp(Node.QUOTED_PROP, false); Node objNameNode = astFactory.createName(objName, objectType); Node access = isQuotedAccess ? astFactory.createGetElem(objNameNode, propdef) : astFactory.createGetProp(objNameNode, propdef.getString(), type(propdef)); result = astFactory.createComma(astFactory.createAssign(access, val), result); } } Node statement = obj; while (!NodeUtil.isStatement(statement)) { statement = statement.getParent(); } result.srcrefTreeIfMissing(obj); obj.replaceWith(result); Node var = IR.var(astFactory.createName(objName, objectType), obj); var.srcrefTreeIfMissing(statement); var.insertBefore(statement); compiler.reportChangeToEnclosingScope(var); } }




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