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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.checkState;
import static com.google.javascript.jscomp.AstFactory.type;

import com.google.common.base.Predicate;
import com.google.common.base.Supplier;
import com.google.common.collect.HashBasedTable;
import com.google.common.collect.HashMultimap;
import com.google.common.collect.LinkedHashMultimap;
import com.google.common.collect.Multimap;
import com.google.common.collect.Table;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
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.Token;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;
import org.jspecify.nullness.Nullable;

/**
 * Rewrite "let"s and "const"s as "var"s. Rename block-scoped declarations and their references when
 * necessary.
 *
 * 

Note that this must run after Es6RewriteDestructuring, since it does not process destructuring * let/const declarations at all. * *

TODO(moz): Try to use MakeDeclaredNamesUnique */ public final class Es6RewriteBlockScopedDeclaration extends AbstractPostOrderCallback implements CompilerPass { private final AbstractCompiler compiler; private final AstFactory astFactory; private final Table renameTable = HashBasedTable.create(); private final Set letConsts = new HashSet<>(); private final Set undeclaredNames = new HashSet<>(); private final Set externNames = new HashSet<>(); private static final FeatureSet transpiledFeatures = FeatureSet.BARE_MINIMUM.with(Feature.LET_DECLARATIONS, Feature.CONST_DECLARATIONS); private final Supplier uniqueNameIdSupplier; public Es6RewriteBlockScopedDeclaration(AbstractCompiler compiler) { this.compiler = compiler; this.uniqueNameIdSupplier = compiler.getUniqueNameIdSupplier(); this.astFactory = compiler.createAstFactory(); } @Override public void visit(NodeTraversal t, Node n, Node parent) { if (!n.hasChildren() || !NodeUtil.isBlockScopedDeclaration(n.getFirstChild())) { return; } // NOTE: This pass depends on for-of being transpiled away before it runs. checkState(parent == null || !parent.isForOf(), parent); if (n.isLet() || n.isConst()) { letConsts.add(n); } if (NodeUtil.isNameDeclaration(n)) { for (Node nameNode = n.getFirstChild(); nameNode != null; nameNode = nameNode.getNext()) { visitBlockScopedName(t, n, nameNode); } } else { // NOTE: This pass depends on class declarations having been transpiled away checkState(n.isFunction() || n.isCatch(), "Unexpected declaration node: %s", n); visitBlockScopedName(t, n, n.getFirstChild()); } } @Override public void process(Node externs, Node root) { NodeTraversal.traverse(compiler, root, new CollectUndeclaredNames()); // Record names declared in externs to prevent collisions when declaring vars from let/const. this.externNames.addAll(NodeUtil.collectExternVariableNames(compiler, externs)); NodeTraversal.traverse(compiler, root, this); NodeTraversal.traverse(compiler, root, new Es6RenameReferences(renameTable)); LoopClosureTransformer transformer = new LoopClosureTransformer(); NodeTraversal.traverse(compiler, root, transformer); transformer.transformLoopClosure(); rewriteDeclsToVars(); TranspilationPasses.maybeMarkFeaturesAsTranspiledAway(compiler, transpiledFeatures); } /** * Renames block-scoped declarations that shadow a variable in an outer scope * *

Also normalizes declarations with no initializer in a loop to be initialized to undefined. */ private void visitBlockScopedName(NodeTraversal t, Node decl, Node nameNode) { Scope scope = t.getScope(); Node parent = decl.getParent(); // Normalize "let x;" to "let x = undefined;" if in a loop, since we later convert x // to be $jscomp$loop$0.x and want to reset the property to undefined every loop iteration. if ((decl.isLet() || decl.isConst()) && !nameNode.hasChildren() && (parent == null || !parent.isForIn()) && inLoop(decl)) { Node undefined = astFactory.createUndefinedValue().srcrefTree(nameNode); nameNode.addChildToFront(undefined); compiler.reportChangeToEnclosingScope(undefined); } String oldName = nameNode.getString(); Scope hoistScope = scope.getClosestHoistScope(); if (scope != hoistScope) { String newName = oldName; if (hoistScope.hasSlot(oldName) || undeclaredNames.contains(oldName) || externNames.contains(oldName)) { do { newName = oldName + "$" + compiler.getUniqueNameIdSupplier().get(); } while (hoistScope.hasSlot(newName)); nameNode.setString(newName); compiler.reportChangeToEnclosingScope(nameNode); Node scopeRoot = scope.getRootNode(); renameTable.put(scopeRoot, oldName, newName); } Var oldVar = scope.getVar(oldName); scope.undeclare(oldVar); hoistScope.declare(newName, nameNode, oldVar.getInput()); } } /** * Whether n is inside a loop. If n is inside a function which is inside a loop, we do not * consider it to be inside a loop. */ private boolean inLoop(Node n) { Node enclosingNode = NodeUtil.getEnclosingNode(n, isLoopOrFunction); return enclosingNode != null && !enclosingNode.isFunction(); } private static final Predicate isLoopOrFunction = new Predicate() { @Override public boolean apply(Node n) { return n.isFunction() || NodeUtil.isLoopStructure(n); } }; private static void extractInlineJSDoc(Node srcDeclaration, Node srcName, Node destDeclaration) { JSDocInfo existingInfo = srcDeclaration.getJSDocInfo(); if (existingInfo == null) { // Extract inline JSDoc from "src" and add it to the "dest" node. existingInfo = srcName.getJSDocInfo(); srcName.setJSDocInfo(null); } JSDocInfo.Builder builder = JSDocInfo.Builder.maybeCopyFrom(existingInfo); destDeclaration.setJSDocInfo(builder.build()); } private static void maybeAddConstJSDoc(Node srcDeclaration, Node srcName, Node destDeclaration) { if (srcDeclaration.isConst()) { extractInlineJSDoc(srcDeclaration, srcName, destDeclaration); JSDocInfo.Builder builder = JSDocInfo.Builder.maybeCopyFrom(destDeclaration.getJSDocInfo()); builder.recordConstancy(); destDeclaration.setJSDocInfo(builder.build()); } } private void handleDeclarationList(Node declarationList, Node parent) { // Normalize: "const i = 0, j = 0;" becomes "/** @const */ var i = 0; /** @const */ var j = 0;" while (declarationList.hasMoreThanOneChild()) { Node name = declarationList.getLastChild(); Node newDeclaration = IR.var(name.detach()).srcref(declarationList); maybeAddConstJSDoc(declarationList, name, newDeclaration); newDeclaration.insertAfter(declarationList); compiler.reportChangeToEnclosingScope(parent); } maybeAddConstJSDoc(declarationList, declarationList.getFirstChild(), declarationList); declarationList.setToken(Token.VAR); } private void addNodeBeforeLoop(Node newNode, Node loopNode) { Node insertSpot = loopNode; while (insertSpot.getParent().isLabel()) { insertSpot = insertSpot.getParent(); } newNode.insertBefore(insertSpot); compiler.reportChangeToEnclosingScope(newNode); } private void rewriteDeclsToVars() { if (!letConsts.isEmpty()) { for (Node n : letConsts) { if (n.isConst()) { handleDeclarationList(n, n.getParent()); } n.setToken(Token.VAR); compiler.reportChangeToEnclosingScope(n); } } } /** * Records undeclared names and aggressively rename possible references to them. Eg: In "{ let * inner; } use(inner);", we rename the let declared variable. */ private class CollectUndeclaredNames extends AbstractPostOrderCallback { @Override public void visit(NodeTraversal t, Node n, Node parent) { if (n.isName() && !t.getScope().hasSlot(n.getString())) { undeclaredNames.add(n.getString()); } } } /** Transforms let/const declarations captured by loop closures. */ private class LoopClosureTransformer extends AbstractPostOrderCallback { private static final String LOOP_OBJECT_NAME = "$jscomp$loop"; private static final String LOOP_OBJECT_PROPERTY_NAME = "$jscomp$loop$prop$"; private final Map loopObjectMap = new LinkedHashMap<>(); private final Multimap functionLoopObjectsMap = LinkedHashMultimap.create(); private final Multimap functionHandledMap = HashMultimap.create(); private final Multimap referenceMap = LinkedHashMultimap.create(); // Maps from a var to a unique property name for that var // e.g. 'i' -> '$jscomp$loop$prop$i$0' private final Map propertyNameMap = new LinkedHashMap<>(); @Override public void visit(NodeTraversal t, Node n, Node parent) { if (!NodeUtil.isReferenceName(n)) { return; } String name = n.getString(); Scope referencedIn = t.getScope(); Var var = referencedIn.getVar(name); if (var == null) { return; } if (!var.isLet() && !var.isConst()) { return; } if (n.getParent().isLet() || n.getParent().isConst()) { letConsts.add(n.getParent()); } // Traverse nodes up from let/const declaration: // If we hit a function or the root before a loop - Not a loop closure. // if we hit a loop first - maybe loop closure. Scope declaredIn = var.getScope(); Node loopNode = null; for (Scope s = declaredIn; ; s = s.getParent()) { Node scopeRoot = s.getRootNode(); if (NodeUtil.isLoopStructure(scopeRoot)) { loopNode = scopeRoot; break; } else if (scopeRoot.hasParent() && NodeUtil.isLoopStructure(scopeRoot.getParent())) { loopNode = scopeRoot.getParent(); break; } else if (s.isFunctionBlockScope() || s.isGlobal()) { return; } } referenceMap.put(var, n); // Traverse scopes from reference scope to declaration scope. // If we hit a function - loop closure detected. Scope outerMostFunctionScope = null; for (Scope s = referencedIn; s != declaredIn && s.getRootNode() != loopNode; s = s.getParent()) { if (s.isFunctionScope()) { outerMostFunctionScope = s; } } if (outerMostFunctionScope != null) { Node function = outerMostFunctionScope.getRootNode(); if (functionHandledMap.containsEntry(function, name)) { return; } functionHandledMap.put(function, name); LoopObject object = loopObjectMap.computeIfAbsent( loopNode, (Node k) -> new LoopObject( LOOP_OBJECT_NAME + "$" + compiler.getUniqueNameIdSupplier().get())); String newPropertyName = createUniquePropertyName(var); object.vars.add(var); propertyNameMap.put(var, newPropertyName); functionLoopObjectsMap.put(function, object); } } private String createUniquePropertyName(Var var) { return LOOP_OBJECT_PROPERTY_NAME + var.getName() + "$" + uniqueNameIdSupplier.get(); } private void transformLoopClosure() { if (loopObjectMap.isEmpty()) { return; } for (Node loopNode : loopObjectMap.keySet()) { // Introduce objects to reflect the captured scope variables. // Fields are initially left as undefined to avoid cases like: // var $jscomp$loop$0 = {$jscomp$loop$prop$i: 0, $jscomp$loop$prop$j: $jscomp$loop$0.i} // They are initialized lazily by changing declarations into assignments // later. LoopObject loopObject = loopObjectMap.get(loopNode); Node objectLitNextIteration = astFactory.createObjectLit(); for (Var var : loopObject.vars) { String newPropertyName = propertyNameMap.get(var); objectLitNextIteration.addChildToBack( astFactory.createStringKey( newPropertyName, createLoopVarReferenceReplacement( loopObject, var.getNameNode(), newPropertyName))); } Node updateLoopObject = astFactory.createAssign(createLoopObjectNameNode(loopObject), objectLitNextIteration); Node objectLit = IR.var(createLoopObjectNameNode(loopObject), astFactory.createObjectLit()) .srcrefTree(loopNode); addNodeBeforeLoop(objectLit, loopNode); if (loopNode.isVanillaFor()) { // For // The initializer is pulled out and placed prior to the loop. Node initializer = loopNode.getFirstChild(); initializer.replaceWith(IR.empty()); if (!initializer.isEmpty()) { if (!NodeUtil.isNameDeclaration(initializer)) { initializer = IR.exprResult(initializer).srcref(initializer); } addNodeBeforeLoop(initializer, loopNode); } Node increment = loopNode.getChildAtIndex(2); if (increment.isEmpty()) { increment.replaceWith(updateLoopObject.srcrefTreeIfMissing(loopNode)); } else { Node placeHolder = IR.empty(); increment.replaceWith(placeHolder); placeHolder.replaceWith( astFactory.createComma(updateLoopObject, increment).srcrefTreeIfMissing(loopNode)); } } else { // We need to make sure the loop object update happens on every loop iteration. // We want to keep it at the end of the loop, because that makes it easier to reason // about the types. // // TODO(bradfordcsmith): Maybe move the update to the start of the loop when this pass // is moved after the type checking passes. // // A finally block would do it, but would have more runtime cost, so instead, if we find // that there are continue statements referring to the loop we will do this. // // originalLoopLabel: while (condition) { // $jscomp$loop$0: { // // original loop body here // // with continue statements converted to `break $jscomp$loop$0;` // // If originalLoopLabel exists, we'll also need to traverse into innner loops // // and convert `continue originalLoopLabel;`. // } // $jscomp$loop$0 = { var1: $jscomp$loop$0.var1, var2: $jscomp$loop$0.var2, ... }; // } // We're intentionally using the same name for the inner loop label and the loop variable // object. Label names and variables are different namespaces, so they do not conflict. String innerBlockLabel = loopObject.name; Node loopBody = NodeUtil.getLoopCodeBlock(loopNode); if (maybeUpdateContinueStatements(loopNode, innerBlockLabel)) { Node innerBlock = IR.block().srcref(loopBody); innerBlock.addChildrenToFront(loopBody.removeChildren()); loopBody.addChildToFront( IR.label(IR.labelName(innerBlockLabel).srcref(loopBody), innerBlock) .srcref(loopBody)); } loopBody.addChildToBack(IR.exprResult(updateLoopObject).srcrefTreeIfMissing(loopNode)); } compiler.reportChangeToEnclosingScope(loopNode); // For captured variables, change declarations to assignments on the // corresponding field of the introduced object. Rename all references // accordingly. for (Var var : loopObject.vars) { String newPropertyName = propertyNameMap.get(var); for (Node reference : referenceMap.get(var)) { // for-of loops are transpiled away before this pass runs checkState(!loopNode.isForOf(), loopNode); // For-of and for-in declarations are not altered, since they are // used as temporary variables for assignment. if (loopNode.isForIn() && loopNode.getFirstChild() == reference.getParent()) { // reference is the node loopVar in a for-in or for-of that looks like this: // `for (const loopVar of list) {` checkState(reference == var.getNameNode(), reference); Node referenceParent = reference.getParent(); checkState(NodeUtil.isNameDeclaration(referenceParent), referenceParent); checkState(reference.isName(), reference); // Start transpiled form of // `for (const p in obj) { ... }` // with this statement to copy the loop variable into the corresponding loop object // property. // `$jscomp$loop$0.$jscomp$loop$prop$0$p = p;` Node loopVarReference = reference.cloneNode(); loopNode .getLastChild() .addChildToFront( IR.exprResult( astFactory.createAssign( createLoopVarReferenceReplacement( loopObject, reference, newPropertyName), loopVarReference)) .srcrefTreeIfMissing(reference)); } else { if (NodeUtil.isNameDeclaration(reference.getParent())) { Node declaration = reference.getParent(); Node grandParent = declaration.getParent(); handleDeclarationList(declaration, grandParent); declaration = reference.getParent(); // Might have changed after normalization. // Change declaration to assignment, or just drop it if there's // no initial value. if (reference.hasChildren()) { Node newReference = cloneWithType(reference); Node assign = astFactory.createAssign(newReference, reference.removeFirstChild()); extractInlineJSDoc(declaration, reference, declaration); maybeAddConstJSDoc(declaration, reference, declaration); assign.setJSDocInfo(declaration.getJSDocInfo()); Node replacement = IR.exprResult(assign).srcrefTreeIfMissing(declaration); declaration.replaceWith(replacement); reference = newReference; } else { declaration.detach(); } letConsts.remove(declaration); compiler.reportChangeToEnclosingScope(grandParent); } if (reference.getParent().isCall() && reference.getParent().getFirstChild() == reference) { reference.getParent().putBooleanProp(Node.FREE_CALL, false); } // Change reference to GETPROP. Node changeScope = NodeUtil.getEnclosingChangeScopeRoot(reference); reference.replaceWith( createLoopVarReferenceReplacement(loopObject, reference, newPropertyName)); // TODO(johnlenz): Don't work on detached nodes. if (changeScope != null) { compiler.reportChangeToChangeScope(changeScope); } } } } } // Create wrapper functions and call them. for (Node function : functionLoopObjectsMap.keySet()) { Node returnNode = IR.returnNode(); Collection objects = functionLoopObjectsMap.get(function); Node[] objectNames = new Node[objects.size()]; Node[] objectNamesForCall = new Node[objects.size()]; int i = 0; for (LoopObject object : objects) { Node paramObjectName = createLoopObjectNameNode(object); objectNames[i] = paramObjectName; objectNamesForCall[i] = createLoopObjectNameNode(object); i++; } Node iife = astFactory.createFunction( "", IR.paramList(objectNames), IR.block(returnNode), type(StandardColors.TOP_OBJECT)); compiler.reportChangeToChangeScope(iife); Node call = astFactory.createCall(iife, type(function), objectNamesForCall); call.putBooleanProp(Node.FREE_CALL, true); Node replacement; if (NodeUtil.isFunctionDeclaration(function)) { replacement = IR.var(IR.name(function.getFirstChild().getString()), call) .srcrefTreeIfMissing(function); } else { replacement = call.srcrefTreeIfMissing(function); } function.replaceWith(replacement); returnNode.addChildToFront(function); compiler.reportChangeToEnclosingScope(replacement); } } /** * Creates a `$jscomp$loop$0.$jscomp$loop$prop$varName$1` replacement for a reference to * `varName`. */ private Node createLoopVarReferenceReplacement( LoopObject loopObject, Node reference, String propertyName) { Node replacement = astFactory.createGetProp( createLoopObjectNameNode(loopObject), propertyName, type(reference)); replacement.srcrefTree(reference); return replacement; } private Node createLoopObjectNameNode(LoopObject loopObject) { return astFactory.createName(loopObject.name, type(StandardColors.TOP_OBJECT)); } /** * Converts all continue statements referring to the given loop to `break $jscomp$loop$0;` where * `$jscomp$loop$0` is the label on the block containing the original loop body. * *

If this method returns {@code true}, then we must wrap the original loop body in a block * labeled with the name from the loopObject. * * @return True if at least one continue statement was found and replaced. */ private boolean maybeUpdateContinueStatements(Node loopNode, String breakLabel) { Node loopParent = loopNode.getParent(); final String originalLoopLabel = loopParent.isLabel() ? loopParent.getFirstChild().getString() : null; ContinueStatementUpdater continueStatementUpdater = new ContinueStatementUpdater(breakLabel, originalLoopLabel); NodeTraversal.traverse( compiler, NodeUtil.getLoopCodeBlock(loopNode), continueStatementUpdater); return continueStatementUpdater.replacedAContinueStatement; } /** * Converts all continue statements referring to the given loop to `break $jscomp$loop$0;` where * `$jscomp$loop$0` is the label on the block containing the original loop body. */ private class ContinueStatementUpdater implements NodeTraversal.Callback { // label to put on break statements created that replace continue statements. private final String breakLabel; private final @Nullable String originalLoopLabel; // Track how many levels of loops deep we go below this one. int loopDepth = 0; // Set to true if a continue statement is found boolean replacedAContinueStatement = false; public ContinueStatementUpdater(String breakLabel, @Nullable String originalLoopLabel) { this.breakLabel = breakLabel; this.originalLoopLabel = originalLoopLabel; } @Override public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) { // NOTE: This pass runs after ES6 classes have already been transpiled away. checkState(!n.isClass(), n); if (n.isFunction()) { return false; } else if (NodeUtil.isLoopStructure(n)) { if (originalLoopLabel == null) { // If this loop has no label, there cannot be any continue statements referring to it // in inner loops. return false; } else { loopDepth++; return true; } } else { return true; } } @Override public void visit(NodeTraversal t, Node n, Node parent) { if (NodeUtil.isLoopStructure(n)) { loopDepth--; } else if (n.isContinue()) { if (loopDepth == 0 && !n.hasChildren()) { replaceWithBreak(n); } else if (originalLoopLabel != null && n.hasChildren() && originalLoopLabel.equals(n.getOnlyChild().getString())) { replaceWithBreak(n); } // else continue belongs to some other loop } // else nothing to do } private void replaceWithBreak(Node continueNode) { Node labelName = IR.labelName(breakLabel).srcref(continueNode); Node breakNode = IR.breakNode(labelName).srcref(continueNode); continueNode.replaceWith(breakNode); replacedAContinueStatement = true; } } private class LoopObject { /** * The name of the variable having the loop's internal variables as properties, and the label * applied to the block containing the original loop body in cases where these are needed. */ private final String name; private final Set vars = new LinkedHashSet<>(); private LoopObject(String name) { this.name = name; } } } private Node cloneWithType(Node node) { Node clone = node.cloneNode(); if (astFactory.isAddingColors()) { clone.setColor(node.getColor()); } return clone; } }





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