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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.
/*
* 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 static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkState;
import static com.google.javascript.jscomp.Es6ToEs3Util.arrayFromIterator;
import static com.google.javascript.jscomp.Es6ToEs3Util.makeIterator;
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.JSDocInfoBuilder;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import com.google.javascript.rhino.TokenStream;
/**
* Rewrites ES6 destructuring patterns and default parameters to valid ES3 code.
*/
public final class Es6RewriteDestructuring implements NodeTraversal.Callback, HotSwapCompilerPass {
private final AbstractCompiler compiler;
private static final FeatureSet transpiledFeatures =
FeatureSet.BARE_MINIMUM.with(Feature.DEFAULT_PARAMETERS, Feature.DESTRUCTURING);
static final String DESTRUCTURING_TEMP_VAR = "$jscomp$destructuring$var";
private int destructuringVarCounter = 0;
public Es6RewriteDestructuring(AbstractCompiler compiler) {
this.compiler = compiler;
}
@Override
public void process(Node externs, Node root) {
TranspilationPasses.processTranspile(compiler, externs, transpiledFeatures, this);
TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this);
TranspilationPasses.markFeaturesAsTranspiledAway(compiler, transpiledFeatures);
}
@Override
public void hotSwapScript(Node scriptRoot, Node originalRoot) {
TranspilationPasses.hotSwapTranspile(compiler, scriptRoot, transpiledFeatures, this);
TranspilationPasses.markFeaturesAsTranspiledAway(compiler, transpiledFeatures);
}
@Override
public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
switch (n.getToken()) {
case FUNCTION:
visitFunction(t, n);
break;
case PARAM_LIST:
visitParamList(n, parent);
break;
case FOR_OF:
visitForOf(n);
break;
default:
break;
}
return true;
}
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (parent != null && parent.isDestructuringLhs()) {
parent = parent.getParent();
}
switch (n.getToken()) {
case ARRAY_PATTERN:
case OBJECT_PATTERN:
visitPattern(t, n, parent);
break;
default:
break;
}
}
/**
* If the function is an arrow function, wrap the body in a block if it is not already a block.
*/
private void visitFunction(NodeTraversal t, Node function) {
Node body = function.getLastChild();
if (!body.isNormalBlock()) {
body.detach();
Node replacement = IR.block(IR.returnNode(body)).useSourceInfoIfMissingFromForTree(body);
function.addChildToBack(replacement);
t.reportCodeChange();
}
}
/**
* Processes trailing default and rest parameters.
*/
private void visitParamList(Node paramList, Node function) {
Node insertSpot = null;
Node body = function.getLastChild();
int i = 0;
Node next = null;
for (Node param = paramList.getFirstChild(); param != null; param = next, i++) {
next = param.getNext();
if (param.isDefaultValue()) {
JSDocInfo jsDoc = param.getJSDocInfo();
Node nameOrPattern = param.removeFirstChild();
Node defaultValue = param.removeFirstChild();
Node newParam;
// Treat name=undefined (and equivalent) as if it was just name. There
// is no need to generate a (name===void 0?void 0:name) statement for
// such arguments.
boolean isNoop = false;
if (!nameOrPattern.isName()) {
// Do not try to optimize unless nameOrPattern is a simple name.
} else if (defaultValue.isName()) {
isNoop = "undefined".equals(defaultValue.getString());
} else if (defaultValue.isVoid()) {
// Any kind of 'void literal' is fine, but 'void fun()' or anything
// else with side effects isn't. We're not trying to be particularly
// smart here and treat 'void {}' for example as if it could cause side effects.
isNoop = NodeUtil.isImmutableValue(defaultValue.getFirstChild());
}
if (isNoop) {
newParam = nameOrPattern.cloneTree();
} else {
newParam =
nameOrPattern.isName()
? nameOrPattern
: IR.name(getTempParameterName(function, i));
Node lhs = nameOrPattern.cloneTree();
Node rhs = defaultValueHook(newParam.cloneTree(), defaultValue);
Node newStatement =
nameOrPattern.isName() ? IR.exprResult(IR.assign(lhs, rhs)) : IR.var(lhs, rhs);
newStatement.useSourceInfoIfMissingFromForTree(param);
body.addChildAfter(newStatement, insertSpot);
insertSpot = newStatement;
}
paramList.replaceChild(param, newParam);
newParam.setOptionalArg(true);
newParam.setJSDocInfo(jsDoc);
compiler.reportChangeToChangeScope(function);
} else if (param.isDestructuringPattern()) {
insertSpot =
replacePatternParamWithTempVar(
function, insertSpot, param, getTempParameterName(function, i));
compiler.reportChangeToChangeScope(function);
} else if (param.isRest() && param.getFirstChild().isDestructuringPattern()) {
insertSpot =
replacePatternParamWithTempVar(
function, insertSpot, param.getFirstChild(), getTempParameterName(function, i));
compiler.reportChangeToChangeScope(function);
}
}
}
/**
* Replace a destructuring pattern parameter with a a temporary parameter name and add a new
* local variable declaration to the function assigning the temporary parameter to the pattern.
*
* Note: Rewrites of variable declaration destructuring will happen later to rewrite
* this declaration as non-destructured code.
* @param function
* @param insertSpot The local variable declaration will be inserted after this statement.
* @param patternParam
* @param tempVarName the name to use for the temporary variable
* @return the declaration statement that was generated for the local variable
*/
private Node replacePatternParamWithTempVar(
Node function, Node insertSpot, Node patternParam, String tempVarName) {
Node newParam = IR.name(tempVarName);
newParam.setJSDocInfo(patternParam.getJSDocInfo());
patternParam.replaceWith(newParam);
Node newDecl = IR.var(patternParam, IR.name(tempVarName));
function.getLastChild().addChildAfter(newDecl, insertSpot);
return newDecl;
}
/**
* Find or create the best name to use for a parameter we need to rewrite.
*
*
* - Use the JS Doc function parameter name at the given index, if possible.
*
- Otherwise, build one of our own.
*
* @param function
* @param parameterIndex
* @return name to use for the given parameter
*/
private String getTempParameterName(Node function, int parameterIndex) {
String tempVarName;
JSDocInfo fnJSDoc = NodeUtil.getBestJSDocInfo(function);
if (fnJSDoc != null && fnJSDoc.getParameterNameAt(parameterIndex) != null) {
tempVarName = fnJSDoc.getParameterNameAt(parameterIndex);
} else {
tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
}
checkState(TokenStream.isJSIdentifier(tempVarName));
return tempVarName;
}
private void visitForOf(Node node) {
Node lhs = node.getFirstChild();
if (lhs.isDestructuringLhs()) {
visitDestructuringPatternInEnhancedFor(lhs.getFirstChild());
}
}
private void visitPattern(NodeTraversal t, Node pattern, Node parent) {
if (NodeUtil.isNameDeclaration(parent) && !NodeUtil.isEnhancedFor(parent.getParent())) {
replacePattern(t, pattern, pattern.getNext(), parent, parent);
} else if (parent.isAssign()) {
if (parent.getParent().isExprResult()) {
replacePattern(t, pattern, pattern.getNext(), parent, parent.getParent());
} else {
wrapAssignmentInCallToArrow(t, parent);
}
} else if (parent.isRest()
|| parent.isStringKey()
|| parent.isArrayPattern()
|| parent.isDefaultValue()) {
// Nested pattern; do nothing. We will visit it after rewriting the parent.
} else if (NodeUtil.isEnhancedFor(parent) || NodeUtil.isEnhancedFor(parent.getParent())) {
visitDestructuringPatternInEnhancedFor(pattern);
} else if (parent.isCatch()) {
visitDestructuringPatternInCatch(pattern);
} else {
throw new IllegalStateException("unexpected parent");
}
}
private void replacePattern(
NodeTraversal t, Node pattern, Node rhs, Node parent, Node nodeToDetach) {
switch (pattern.getToken()) {
case ARRAY_PATTERN:
replaceArrayPattern(t, pattern, rhs, parent, nodeToDetach);
break;
case OBJECT_PATTERN:
replaceObjectPattern(t, pattern, rhs, parent, nodeToDetach);
break;
default:
throw new IllegalStateException("unexpected");
}
}
/**
* Convert 'var {a: b, c: d} = rhs' to:
*
* @const var temp = rhs; var b = temp.a; var d = temp.c;
*/
private void replaceObjectPattern(
NodeTraversal t, Node objectPattern, Node rhs, Node parent, Node nodeToDetach) {
String tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node tempDecl = IR.var(IR.name(tempVarName), rhs.detach())
.useSourceInfoIfMissingFromForTree(objectPattern);
// TODO(tbreisacher): Remove the "if" and add this JSDoc unconditionally.
if (parent.isConst()) {
JSDocInfoBuilder jsDoc = new JSDocInfoBuilder(false);
jsDoc.recordConstancy();
tempDecl.setJSDocInfo(jsDoc.build());
}
nodeToDetach.getParent().addChildBefore(tempDecl, nodeToDetach);
for (Node child = objectPattern.getFirstChild(), next; child != null; child = next) {
next = child.getNext();
Node newLHS;
Node newRHS;
if (child.isStringKey()) {
if (!child.hasChildren()) { // converting shorthand
Node name = IR.name(child.getString());
name.useSourceInfoIfMissingFrom(child);
child.addChildToBack(name);
}
Node getprop =
new Node(
child.isQuotedString() ? Token.GETELEM : Token.GETPROP,
IR.name(tempVarName),
IR.string(child.getString()));
Node value = child.removeFirstChild();
if (!value.isDefaultValue()) {
newLHS = value;
newRHS = getprop;
} else {
newLHS = value.removeFirstChild();
Node defaultValue = value.removeFirstChild();
newRHS = defaultValueHook(getprop, defaultValue);
}
} else if (child.isComputedProp()) {
if (child.getLastChild().isDefaultValue()) {
newLHS = child.getLastChild().removeFirstChild();
Node getelem = IR.getelem(IR.name(tempVarName), child.removeFirstChild());
String intermediateTempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node intermediateDecl = IR.var(IR.name(intermediateTempVarName), getelem);
intermediateDecl.useSourceInfoIfMissingFromForTree(child);
nodeToDetach.getParent().addChildBefore(intermediateDecl, nodeToDetach);
newRHS =
defaultValueHook(
IR.name(intermediateTempVarName), child.getLastChild().removeFirstChild());
} else {
newRHS = IR.getelem(IR.name(tempVarName), child.removeFirstChild());
newLHS = child.removeFirstChild();
}
} else if (child.isDefaultValue()) {
newLHS = child.removeFirstChild();
Node defaultValue = child.removeFirstChild();
Node getprop = IR.getprop(IR.name(tempVarName), IR.string(newLHS.getString()));
newRHS = defaultValueHook(getprop, defaultValue);
} else {
throw new IllegalStateException("unexpected child");
}
Node newNode;
if (NodeUtil.isNameDeclaration(parent)) {
newNode = IR.declaration(newLHS, newRHS, parent.getToken());
} else if (parent.isAssign()) {
newNode = IR.exprResult(IR.assign(newLHS, newRHS));
} else {
throw new IllegalStateException("not reached");
}
newNode.useSourceInfoIfMissingFromForTree(child);
nodeToDetach.getParent().addChildBefore(newNode, nodeToDetach);
// Explicitly visit the LHS of the new node since it may be a nested
// destructuring pattern.
visit(t, newLHS, newLHS.getParent());
}
nodeToDetach.detach();
t.reportCodeChange();
}
/**
* Convert var [x, y] = rhs to:
*
* var temp = $jscomp.makeIterator(rhs);
* var x = temp.next().value;
* var y = temp.next().value;
*
*/
private void replaceArrayPattern(
NodeTraversal t, Node arrayPattern, Node rhs, Node parent, Node nodeToDetach) {
String tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node tempDecl = IR.var(
IR.name(tempVarName),
makeIterator(compiler, rhs.detach()));
tempDecl.useSourceInfoIfMissingFromForTree(arrayPattern);
nodeToDetach.getParent().addChildBefore(tempDecl, nodeToDetach);
for (Node child = arrayPattern.getFirstChild(), next; child != null; child = next) {
next = child.getNext();
if (child.isEmpty()) {
// Just call the next() method to advance the iterator, but throw away the value.
Node nextCall = IR.exprResult(IR.call(IR.getprop(IR.name(tempVarName), IR.string("next"))));
nextCall.useSourceInfoIfMissingFromForTree(child);
nodeToDetach.getParent().addChildBefore(nextCall, nodeToDetach);
continue;
}
Node newLHS;
Node newRHS;
if (child.isDefaultValue()) {
// [x = defaultValue] = rhs;
// becomes
// var temp0 = $jscomp.makeIterator(rhs);
// var temp1 = temp.next().value
// x = (temp1 === undefined) ? defaultValue : temp1;
String nextVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node var = IR.var(
IR.name(nextVarName),
IR.getprop(
IR.call(IR.getprop(IR.name(tempVarName), IR.string("next"))),
IR.string("value")));
var.useSourceInfoIfMissingFromForTree(child);
nodeToDetach.getParent().addChildBefore(var, nodeToDetach);
newLHS = child.getFirstChild().detach();
newRHS = defaultValueHook(IR.name(nextVarName), child.getLastChild().detach());
} else if (child.isRest()) {
// [...x] = rhs;
// becomes
// var temp = $jscomp.makeIterator(rhs);
// x = $jscomp.arrayFromIterator(temp);
newLHS = child.getFirstChild().detach();
newRHS = arrayFromIterator(compiler, IR.name(tempVarName));
} else {
// LHS is just a name (or a nested pattern).
// var [x] = rhs;
// becomes
// var temp = $jscomp.makeIterator(rhs);
// var x = temp.next().value;
newLHS = child.detach();
newRHS = IR.getprop(
IR.call(IR.getprop(IR.name(tempVarName), IR.string("next"))),
IR.string("value"));
}
Node newNode;
if (parent.isAssign()) {
Node assignment = IR.assign(newLHS, newRHS);
newNode = IR.exprResult(assignment);
} else {
newNode = IR.declaration(newLHS, newRHS, parent.getToken());
}
newNode.useSourceInfoIfMissingFromForTree(arrayPattern);
nodeToDetach.getParent().addChildBefore(newNode, nodeToDetach);
// Explicitly visit the LHS of the new node since it may be a nested
// destructuring pattern.
visit(t, newLHS, newLHS.getParent());
}
nodeToDetach.detach();
t.reportCodeChange();
}
/**
* Convert the assignment '[x, y] = rhs' that is used as an expression and not an expr result to:
* (() => let temp0 = rhs; var temp1 = $jscomp.makeIterator(temp0); var x = temp0.next().value;
* var y = temp0.next().value; return temp0; }) And the assignment '{x: a, y: b} = rhs' used as an
* expression and not an expr result to: (() => let temp0 = rhs; var temp1 = temp0; var a =
* temp0.x; var b = temp0.y; return temp0; })
*/
private void wrapAssignmentInCallToArrow(NodeTraversal t, Node assignment) {
String tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node rhs = assignment.getLastChild().detach();
Node newAssignment = IR.let(IR.name(tempVarName), rhs);
Node replacementExpr = IR.assign(assignment.getFirstChild().detach(), IR.name(tempVarName));
Node exprResult = IR.exprResult(replacementExpr);
Node returnNode = IR.returnNode(IR.name(tempVarName));
Node block = IR.block(newAssignment, exprResult, returnNode);
Node call = IR.call(IR.arrowFunction(IR.name(""), IR.paramList(), block));
call.useSourceInfoIfMissingFromForTree(assignment);
call.putBooleanProp(Node.FREE_CALL, true);
assignment.getParent().replaceChild(assignment, call);
NodeUtil.markNewScopesChanged(call, compiler);
replacePattern(
t,
replacementExpr.getFirstChild(),
replacementExpr.getLastChild(),
replacementExpr,
exprResult);
}
private void visitDestructuringPatternInEnhancedFor(Node pattern) {
checkArgument(pattern.isDestructuringPattern());
String tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
if (NodeUtil.isEnhancedFor(pattern.getParent())) {
Node forNode = pattern.getParent();
Node block = forNode.getLastChild();
Node decl = IR.var(IR.name(tempVarName));
decl.useSourceInfoIfMissingFromForTree(pattern);
forNode.replaceChild(pattern, decl);
Node exprResult = IR.exprResult(IR.assign(pattern, IR.name(tempVarName)));
exprResult.useSourceInfoIfMissingFromForTree(pattern);
block.addChildToFront(exprResult);
} else {
Node destructuringLhs = pattern.getParent();
checkState(destructuringLhs.isDestructuringLhs());
Node declarationNode = destructuringLhs.getParent();
Node forNode = declarationNode.getParent();
checkState(NodeUtil.isEnhancedFor(forNode));
Node block = forNode.getLastChild();
declarationNode.replaceChild(
destructuringLhs, IR.name(tempVarName).useSourceInfoFrom(pattern));
Token declarationType = declarationNode.getToken();
Node decl = IR.declaration(pattern.detach(), IR.name(tempVarName), declarationType);
decl.useSourceInfoIfMissingFromForTree(pattern);
block.addChildToFront(decl);
}
}
private void visitDestructuringPatternInCatch(Node pattern) {
String tempVarName = DESTRUCTURING_TEMP_VAR + (destructuringVarCounter++);
Node catchBlock = pattern.getNext();
pattern.replaceWith(IR.name(tempVarName));
catchBlock.addChildToFront(IR.declaration(pattern, IR.name(tempVarName), Token.LET));
}
/**
* Helper for transpiling DEFAULT_VALUE trees.
*/
private static Node defaultValueHook(Node getprop, Node defaultValue) {
return IR.hook(IR.sheq(getprop, IR.name("undefined")), defaultValue, getprop.cloneTree());
}
}