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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 2008 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.collect.ImmutableSet;
import com.google.common.collect.Sets;
import com.google.javascript.jscomp.AbstractScope.ImplicitVar;
import com.google.javascript.jscomp.NodeTraversal.AbstractShallowCallback;
import com.google.javascript.jscomp.ReferenceCollector.Behavior;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.QualifiedName;
import com.google.javascript.rhino.Token;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import org.jspecify.nullness.Nullable;
/**
* Checks variables to see if they are referenced before their declaration, or if they are
* redeclared in a way that is suspicious (i.e. not dictated by control structures). This is a more
* aggressive version of {@link VarCheck}, but it lacks the cross-module checks.
*/
class VariableReferenceCheck implements CompilerPass {
static final DiagnosticType EARLY_REFERENCE =
DiagnosticType.warning(
"JSC_REFERENCE_BEFORE_DECLARE", "Variable referenced before declaration: {0}");
static final DiagnosticType EARLY_EXPORTS_REFERENCE =
DiagnosticType.error(
"JSC_EXPORTS_REFERENCE_BEFORE_ASSIGN",
"Illegal reference to `exports` before assignment `exports = ...`");
static final DiagnosticType REDECLARED_VARIABLE =
DiagnosticType.warning("JSC_REDECLARED_VARIABLE", "Redeclared variable: {0}");
static final DiagnosticType EARLY_REFERENCE_ERROR =
DiagnosticType.error(
"JSC_REFERENCE_BEFORE_DECLARE_ERROR",
"Illegal variable reference before declaration: {0}");
static final DiagnosticType REASSIGNED_CONSTANT =
DiagnosticType.error("JSC_REASSIGNED_CONSTANT", "Constant reassigned: {0}");
static final DiagnosticType REDECLARED_VARIABLE_ERROR =
DiagnosticType.error("JSC_REDECLARED_VARIABLE_ERROR", "Illegal redeclared variable: {0}");
static final DiagnosticType DECLARATION_NOT_DIRECTLY_IN_BLOCK =
DiagnosticType.error(
"JSC_DECLARATION_NOT_DIRECTLY_IN_BLOCK",
"Block-scoped declaration not directly within block: {0}");
static final DiagnosticType UNUSED_LOCAL_ASSIGNMENT =
DiagnosticType.disabled(
"JSC_UNUSED_LOCAL_ASSIGNMENT", "Value assigned to local variable {0} is never read");
private static final QualifiedName GOOG_REQUIRE = QualifiedName.of("goog.require");
private static final QualifiedName GOOG_REQUIRE_TYPE = QualifiedName.of("goog.requireType");
private static final QualifiedName GOOG_FORWARD_DECLARE = QualifiedName.of("goog.forwardDeclare");
private final AbstractCompiler compiler;
private final boolean checkUnusedLocals;
// NOTE(nicksantos): It's a lot faster to use a shared Set that
// we clear after each method call, because the Set never gets too big.
private final Set blocksWithDeclarations = new HashSet<>();
// These types do not permit a block-scoped declaration inside them without an explicit block.
// e.g. if (b) let x;
// This list omits Token.LABEL intentionally. It's handled differently in IRFactory.
private static final ImmutableSet BLOCKLESS_DECLARATION_FORBIDDEN_STATEMENTS =
Sets.immutableEnumSet(
Token.IF, Token.FOR, Token.FOR_IN, Token.FOR_OF, Token.FOR_AWAIT_OF, Token.WHILE);
private static final QualifiedName GOOG_SCOPE = QualifiedName.of("goog.scope");
public VariableReferenceCheck(AbstractCompiler compiler) {
this.compiler = compiler;
this.checkUnusedLocals =
compiler.getOptions().enables(DiagnosticGroup.forType(UNUSED_LOCAL_ASSIGNMENT));
}
@Override
public void process(Node externs, Node root) {
new ReferenceCollector(
compiler, new ReferenceCheckingBehavior(), new SyntacticScopeCreator(compiler))
.process(externs, root);
}
/**
* Behavior that checks variables for redeclaration or early references just after they go out of
* scope.
*/
private class ReferenceCheckingBehavior implements Behavior {
private final Set varsInFunctionBody;
private ReferenceCheckingBehavior() {
varsInFunctionBody = new HashSet<>();
}
@Override
public void afterExitScope(NodeTraversal t, ReferenceMap referenceMap) {
// Check all vars after finishing a scope
Scope scope = t.getScope();
if (scope.isFunctionBlockScope()) {
varsInFunctionBody.clear();
for (Var v : scope.getVarIterable()) {
varsInFunctionBody.add(v.getName());
}
}
for (Var v : scope.getVarIterable()) {
ReferenceCollection referenceCollection = referenceMap.getReferences(v);
// TODO(moz): Figure out why this could be null
if (referenceCollection != null) {
if (scope.getRootNode().isFunction() && v.isDefaultParam()) {
checkDefaultParam(v, scope, varsInFunctionBody);
}
if (scope.getRootNode().isFunction()) {
checkShadowParam(v, scope, referenceCollection.references);
}
checkVar(v, referenceCollection.references);
}
}
if (scope.hasOwnImplicitSlot(ImplicitVar.EXPORTS)) {
checkGoogModuleExports(scope.makeImplicitVar(ImplicitVar.EXPORTS), referenceMap);
}
}
private void checkDefaultParam(
Var param, final Scope scope, final Set varsInFunctionBody) {
NodeTraversal.traverse(
compiler,
param.getParentNode().getSecondChild(),
/**
* Do a shallow check since cases like: {@code
* function f(y = () => x, x = 5) { return y(); }
* } is legal. We are going to miss cases like: {@code
* function f(y = (() => x)(), x = 5) { return y(); }
* } but this should be rare.
*/
new AbstractShallowCallback() {
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
if (!NodeUtil.isReferenceName(n)) {
return;
}
String refName = n.getString();
if (varsInFunctionBody.contains(refName) && !scope.hasSlot(refName)) {
compiler.report(JSError.make(n, EARLY_REFERENCE_ERROR, refName));
}
}
});
}
private void checkShadowParam(Var v, Scope functionScope, List references) {
Var maybeParam = functionScope.getVar(v.getName());
if (maybeParam != null && maybeParam.isParam() && maybeParam.getScope() == functionScope) {
for (Reference r : references) {
if ((r.isVarDeclaration() || r.isHoistedFunction())
&& r.getNode() != v.getNameNode()) {
compiler.report(JSError.make(r.getNode(), REDECLARED_VARIABLE, v.getName()));
}
}
}
}
private void checkGoogModuleExports(Var exportsVar, ReferenceMap referenceMap) {
ReferenceCollection references = referenceMap.getReferences(exportsVar);
if (references == null || references.isNeverAssigned()) {
return;
}
for (Reference reference : references.references) {
if (reference.isLvalue()) {
break;
}
checkEarlyReference(exportsVar, reference, reference.getNode());
}
}
/**
* If the variable is declared more than once in a basic block, generate a
* warning. Also check if a variable is used in a given scope before it is
* declared, which suggest a likely error. Relies on the fact that
* references is in parse-tree order.
*/
private void checkVar(Var v, List references) {
blocksWithDeclarations.clear();
boolean hasSeenDeclaration = false;
boolean hasErrors = false;
boolean isRead = false;
Reference unusedAssignment = null;
Reference hoistedFn = lookForHoistedFunction(references);
if (hoistedFn != null) {
hasSeenDeclaration = true;
}
for (Reference reference : references) {
if (reference == hoistedFn) {
continue;
}
Node referenceNode = reference.getNode();
BasicBlock basicBlock = reference.getBasicBlock();
boolean isDeclaration = reference.isDeclaration();
boolean isAssignment = isDeclaration || reference.isLvalue();
if (isDeclaration) {
// Checks for declarations
hasSeenDeclaration = true;
hasErrors = checkRedeclaration(v, reference, referenceNode, hoistedFn, basicBlock);
// Add the current basic block after checking redeclarations
blocksWithDeclarations.add(basicBlock);
checkBlocklessDeclaration(v, reference, referenceNode);
if (reference.getGrandparent().isExport()) {
isRead = true;
}
} else {
// Checks for references
if (!hasSeenDeclaration) {
hasErrors = checkEarlyReference(v, reference, referenceNode);
}
if (!hasErrors && v.isConst() && reference.isLvalue()) {
compiler.report(JSError.make(referenceNode, REASSIGNED_CONSTANT, v.getName()));
}
// Check for temporal dead zone of let / const declarations in for-in and for-of loops
// TODO(b/111441110): Fix this check. it causes spurious warnings on `b = a` in
// for (const [a, b = a] of []) {}
if ((v.isLet() || v.isConst())
&& v.getScope() == reference.getScope()
&& NodeUtil.isEnhancedFor(reference.getScope().getRootNode())) {
compiler.report(JSError.make(referenceNode, EARLY_REFERENCE_ERROR, v.getName()));
}
}
if (isAssignment) {
Reference decl = references.get(0);
Node declNode = decl.getNode();
Node gp = declNode.getGrandparent();
boolean lhsOfForInLoop = gp.isForIn() && gp.getFirstFirstChild() == declNode;
if (decl.getScope().isLocal()
&& (decl.isVarDeclaration() || decl.isLetDeclaration() || decl.isConstDeclaration())
&& !decl.getNode().isFromExterns()
&& !lhsOfForInLoop) {
unusedAssignment = reference;
}
if ((reference.getParent().isDec() || reference.getParent().isInc()
|| NodeUtil.isCompoundAssignmentOp(reference.getParent()))
&& NodeUtil.isExpressionResultUsed(reference.getNode())) {
isRead = true;
}
} else {
isRead = true;
}
}
if (checkUnusedLocals && unusedAssignment != null && !isRead && !hasErrors) {
checkForUnusedLocalVar(v, unusedAssignment);
}
}
}
/**
* @return The reference to the hoisted function, if the variable is one
*/
private @Nullable Reference lookForHoistedFunction(List references) {
for (Reference reference : references) {
if (reference.isHoistedFunction()) {
blocksWithDeclarations.add(reference.getBasicBlock());
return reference;
}
}
return null;
}
private void checkBlocklessDeclaration(Var v, Reference reference, Node referenceNode) {
if (!reference.isVarDeclaration() && reference.getGrandparent().isAddedBlock()
&& BLOCKLESS_DECLARATION_FORBIDDEN_STATEMENTS.contains(
reference.getGrandparent().getParent().getToken())) {
compiler.report(JSError.make(referenceNode, DECLARATION_NOT_DIRECTLY_IN_BLOCK, v.getName()));
}
}
/**
* @return If a redeclaration error has been found
*/
private boolean checkRedeclaration(
Var v, Reference reference, Node referenceNode, Reference hoistedFn, BasicBlock basicBlock) {
boolean letConstShadowsVar = v.getParentNode().isVar()
&& (reference.isLetDeclaration() || reference.isConstDeclaration());
boolean isVarNodeSameAsReferenceNode = v.getNode() == reference.getNode();
// We disallow redeclaration of caught exceptions
boolean shadowCatchVar = v.getParentNode().isCatch() && !isVarNodeSameAsReferenceNode;
if (isRedeclaration(basicBlock)
&& !VarCheck.hasDuplicateDeclarationSuppression(compiler, referenceNode, v.getNameNode())) {
final DiagnosticType diagnosticType;
Node warningNode = referenceNode;
boolean shadowParam =
v.isParam()
&& NodeUtil.isBlockScopedDeclaration(referenceNode)
&& v.getScope() == reference.getScope().getParent();
if (v.isLet()
|| v.isConst()
|| v.isClass()
|| letConstShadowsVar
|| shadowCatchVar
|| shadowParam
|| v.isImport()) {
// These cases are all hard errors that violate ES6 semantics
diagnosticType = REDECLARED_VARIABLE_ERROR;
} else if (reference.getNode().getParent().isCatch()) {
return false;
} else {
// These diagnostics are for valid, but suspicious, code, and are suppressible.
// For vars defined in the global scope, give the same error as VarCheck
diagnosticType =
v.getScope().isGlobal() ? VarCheck.VAR_MULTIPLY_DECLARED_ERROR : REDECLARED_VARIABLE;
// Since we skip hoisted functions, we would have the wrong warning node in cases
// where the redeclaration is a function declaration. Check for that case.
if (isVarNodeSameAsReferenceNode
&& hoistedFn != null
&& v.getName().equals(hoistedFn.getNode().getString())) {
warningNode = hoistedFn.getNode();
}
}
compiler.report(
JSError.make(warningNode, diagnosticType, v.getName(), locationOf(v.getNode())));
return true;
}
if ((letConstShadowsVar || shadowCatchVar) && v.getScope() == reference.getScope()) {
compiler.report(JSError.make(referenceNode, REDECLARED_VARIABLE_ERROR, v.getName()));
return true;
}
return false;
}
/**
* Returns whether the given block executes after any known declarations of the variable being
* visited.
*/
private boolean isRedeclaration(BasicBlock newDeclaration) {
for (BasicBlock previousDeclaration : blocksWithDeclarations) {
if (previousDeclaration.provablyExecutesBefore(newDeclaration)) {
return true;
}
}
return false;
}
private static String locationOf(@Nullable Node n) {
return (n == null) ? "" : n.getLocation();
}
/**
* @return If an early reference has been found
*/
private boolean checkEarlyReference(Var v, Reference reference, Node referenceNode) {
// Don't check the order of references in externs files.
if (referenceNode.isFromExterns() || v.isImplicitGoogNamespace()) {
return false;
}
// Special case to deal with var goog = goog || {}. Note that
// let x = x || {} is illegal, just like var y = x || {}; let x = y;
if (v.isVar()) {
Node curr = reference.getParent();
while (curr.isOr() && curr.getParent().getFirstChild() == curr) {
curr = curr.getParent();
}
if (curr.isName() && curr.getString().equals(v.getName())) {
return false;
}
}
// Only generate warnings for early references in the same function scope/global scope in
// order to deal with possible forward declarations and recursion
// e.g. don't warn on:
// function f() { return x; } f(); let x = 5;
// We don't track where `f` is called, just where it's defined, and don't want to warn for
// function f() { return x; } let x = 5; f();
// TODO(moz): See if we can remove the bypass for "goog"
if (reference.getScope().hasSameContainerScope(v.getScope()) && !v.getName().equals("goog")) {
compiler.report(
JSError.make(
reference.getNode(),
v.isGoogModuleExports()
? EARLY_EXPORTS_REFERENCE
: (v.isLet() || v.isConst() || v.isClass() || v.isParam())
? EARLY_REFERENCE_ERROR
: EARLY_REFERENCE,
v.getName()));
return true;
}
return false;
}
// Only check for unused local if not in a goog.scope function.
// TODO(tbreisacher): Consider moving UNUSED_LOCAL_ASSIGNMENT into its own check pass, so
// that we can run it after goog.scope processing, and get rid of the inGoogScope check.
private void checkForUnusedLocalVar(Var v, Reference unusedAssignment) {
if (!v.isLocal()) {
return;
}
JSDocInfo jsDoc = NodeUtil.getBestJSDocInfo(unusedAssignment.getNode());
if (jsDoc != null && jsDoc.hasTypedefType()) {
return;
}
boolean inGoogScope = false;
Scope s = v.getScope();
if (s.isFunctionBlockScope()) {
Node function = s.getRootNode().getParent();
Node callee = function.getPrevious();
inGoogScope = callee != null && GOOG_SCOPE.matches(callee);
}
if (inGoogScope) {
// No warning.
return;
}
if (s.isModuleScope()) {
Node statement = NodeUtil.getEnclosingStatement(v.getNode());
if (NodeUtil.isNameDeclaration(statement)) {
Node lhs = statement.getFirstChild();
Node rhs = lhs.getFirstChild();
if (rhs != null
&& (NodeUtil.isCallTo(rhs, GOOG_FORWARD_DECLARE)
|| NodeUtil.isCallTo(rhs, GOOG_REQUIRE_TYPE)
|| NodeUtil.isCallTo(rhs, GOOG_REQUIRE)
|| rhs.isQualifiedName())) {
// No warning. module imports will be caught by the unused-require check, and if the
// right side is a qualified name then this is likely an alias used in type annotations.
return;
}
}
}
compiler.report(JSError.make(unusedAssignment.getNode(), UNUSED_LOCAL_ASSIGNMENT, v.getName()));
}
}