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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 2012 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 com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.base.Tri;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.jstype.JSType;
/**
* Checks for common errors, such as misplaced semicolons:
*
*
* if (x); act_now();
*
*
* or comparison against NaN:
*
*
* if (x === NaN) act();
*
*
* and generates warnings.
*/
final class CheckSuspiciousCode extends AbstractPostOrderCallback {
static final DiagnosticType SUSPICIOUS_SEMICOLON =
DiagnosticType.warning(
"JSC_SUSPICIOUS_SEMICOLON",
"If this if/for/while really shouldn''t have a body, use '{}'");
static final DiagnosticType SUSPICIOUS_COMPARISON_WITH_NAN =
DiagnosticType.warning(
"JSC_SUSPICIOUS_NAN", "Comparison against NaN is always false. Did you mean isNaN()?");
static final DiagnosticType SUSPICIOUS_IN_OPERATOR =
DiagnosticType.warning(
"JSC_SUSPICIOUS_IN",
"Use of the \"in\" keyword on non-object types throws an exception.");
static final DiagnosticType SUSPICIOUS_INSTANCEOF_LEFT_OPERAND =
DiagnosticType.warning(
"JSC_SUSPICIOUS_INSTANCEOF_LEFT",
"\"instanceof\" with left non-object operand is always false.");
static final DiagnosticType SUSPICIOUS_LEFT_OPERAND_OF_LOGICAL_OPERATOR =
DiagnosticType.warning(
"JSC_SUSPICIOUS_LEFT_OPERAND_OF_LOGICAL_OPERATOR",
"Left operand of {0} operator is always {1}.");
static final DiagnosticType SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR =
DiagnosticType.warning(
"JSC_SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR",
"Suspicious negated left operand of 'in' operator.");
static final DiagnosticType SUSPICIOUS_BREAKING_OUT_OF_OPTIONAL_CHAIN =
DiagnosticType.warning(
"SUSPICIOUS_BREAKING_OUT_OF_OPTIONAL_CHAIN",
"Suspicious breaking out of optional chain. May result in TypeError if optional chain is"
+ " undefined.");
@Override
public void visit(NodeTraversal t, Node n, Node parent) {
checkMissingSemicolon(t, n);
checkNaN(t, n);
checkInvalidIn(t, n);
checkNonObjectInstanceOf(t, n);
checkNegatedLeftOperandOfInOperator(t, n);
checkLeftOperandOfLogicalOperator(t, n);
checkSuspiciousBreakingOutOfOptionalChain(t, n);
}
private void checkMissingSemicolon(NodeTraversal t, Node n) {
switch (n.getToken()) {
case IF:
Node trueCase = n.getSecondChild();
reportIfWasEmpty(t, trueCase);
Node elseCase = trueCase.getNext();
if (elseCase != null) {
reportIfWasEmpty(t, elseCase);
}
break;
case WHILE:
case FOR:
case FOR_IN:
case FOR_OF:
case FOR_AWAIT_OF:
reportIfWasEmpty(t, NodeUtil.getLoopCodeBlock(n));
break;
default:
break;
}
}
private static void reportIfWasEmpty(NodeTraversal t, Node block) {
checkState(block.isBlock());
// A semicolon is distinguished from a block without children by
// annotating it with EMPTY_BLOCK. Blocks without children are
// usually intentional, especially with loops.
if (!block.hasChildren() && block.isAddedBlock()) {
t.getCompiler().report(JSError.make(block, SUSPICIOUS_SEMICOLON));
}
}
private void checkNaN(NodeTraversal t, Node n) {
switch (n.getToken()) {
case EQ:
case GE:
case GT:
case LE:
case LT:
case NE:
case SHEQ:
case SHNE:
reportIfNaN(t, n.getFirstChild());
reportIfNaN(t, n.getLastChild());
break;
default:
break;
}
}
private static void reportIfNaN(NodeTraversal t, Node n) {
if (NodeUtil.isNaN(n)) {
t.getCompiler().report(JSError.make(n.getParent(), SUSPICIOUS_COMPARISON_WITH_NAN));
}
}
private void checkInvalidIn(NodeTraversal t, Node n) {
if (n.isIn()) {
reportIfNonObject(t, n.getLastChild(), SUSPICIOUS_IN_OPERATOR);
}
}
private void checkNonObjectInstanceOf(NodeTraversal t, Node n) {
if (n.isInstanceOf()) {
reportIfNonObject(t, n.getFirstChild(), SUSPICIOUS_INSTANCEOF_LEFT_OPERAND);
}
}
private static boolean reportIfNonObject(NodeTraversal t, Node n, DiagnosticType diagnosticType) {
if (n.isAdd() || !NodeUtil.mayBeObject(n)) {
t.report(n.getParent(), diagnosticType);
return true;
}
return false;
}
private void checkNegatedLeftOperandOfInOperator(NodeTraversal t, Node n) {
if (n.isIn() && n.getFirstChild().isNot()) {
t.report(n.getFirstChild(), SUSPICIOUS_NEGATED_LEFT_OPERAND_OF_IN_OPERATOR);
}
}
/**
* Check for the LHS of a logical operator (&& and ||) being deterministically truthy or
* falsy, using both syntactic and type information. This is always suspicious (though for
* different reasons: "truthy and" means the LHS is always ignored and should be removed, "falsy
* and" means the RHS is dead code, and vice versa for "or". However, there are a number of
* legitimate use cases where we need to back off from using type information (see {@link
* #getBooleanValueWithTypes} for more details on these back offs).
*/
private void checkLeftOperandOfLogicalOperator(NodeTraversal t, Node n) {
if (n.isOr() || n.isAnd()) {
String operator = n.isOr() ? "||" : "&&";
Tri v = getBooleanValueWithTypes(n.getFirstChild());
if (v != Tri.UNKNOWN) {
String result = v == Tri.TRUE ? "truthy" : "falsy";
t.report(n, SUSPICIOUS_LEFT_OPERAND_OF_LOGICAL_OPERATOR, operator, result);
}
}
}
/**
* Checks for breaking out of optional chain. (e.g. `(a?.b).c)` There is no reason to break out of
* optional chains and doing so may cause a TypeError if `a` is nullish. Using `a?.b.c` is safer
* and will have the same effect when `a` is not nullish.
*/
private static void checkSuspiciousBreakingOutOfOptionalChain(NodeTraversal t, Node n) {
if (NodeUtil.isOptChainNode(n)) {
Node parent = n.getParent();
if (n.isFirstChildOf(parent)
&& (parent.isGetProp() || parent.isGetElem() || parent.isCall())) {
t.report(n, SUSPICIOUS_BREAKING_OUT_OF_OPTIONAL_CHAIN);
}
}
}
/**
* Returns the possible boolean values of a node. This is a combination of {@link
* NodeUtil#getBooleanValue} and {@link JSType#getPossibleToBooleanOutcomes}, with some additional
* backoff for situations that are known to be less accurate (to wit, qualified names and truthy
* return types, which return UNKNOWN even if the type information seems conclusive). Another
* difference from the NodeUtil and JSTyoe methods is that we include some specific optimization
* to avoid quadratic behavior of large nested logical operations.
*
* The specifics of when this method returns UNKNOWN instead of TRUE or FALSE is as follows:
*
*
* - We should not determine that a qualified name (expanded slightly to include computed
* properties) is always truthy or always falsy. There are many cases where an extern
* defines a name to be truthy, but it still makes sense to feature-test in the browser.
*
- We should not determine that a getelem or a function call is always truthy (though we
* expand it to simply never complain about always-truthy based only on types) since the
* standard externs lie about the return type of {@code Map.prototype.get} and array
* accesses, which can both return undefined despite what the externs say.
*
*
* We do not back off from boolean literals (e.g. "{@code true &&}"), though they appear to be
* common in generated code. Instead, such code should suppress "suspiciousCode". We also do not
* back off from always-falsy function call results, since it provides a valuable check and lies
* in this direction are much less common.
*/
private Tri getBooleanValueWithTypes(Node n) {
switch (n.getToken()) {
case ASSIGN:
case COMMA:
return getBooleanValueWithTypes(n.getLastChild());
case NOT:
return getBooleanValueWithTypes(n.getLastChild()).not();
case AND:
// Assume the left-hand side is unknown. If it's not then we'll report it elsewhere. This
// prevents revisiting deeper nodes repeatedly, which would result in O(n^2) performance.
return Tri.UNKNOWN.and(getBooleanValueWithTypes(n.getLastChild()));
case OR:
// Assume the left-hand side is unknown. If it's not then we'll report it elsewhere. This
// prevents revisiting deeper nodes repeatedly, which would result in O(n^2) performance.
return Tri.UNKNOWN.or(getBooleanValueWithTypes(n.getLastChild()));
case HOOK:
{
Tri trueValue = getBooleanValueWithTypes(n.getSecondChild());
Tri falseValue = getBooleanValueWithTypes(n.getLastChild());
return trueValue.equals(falseValue) ? trueValue : Tri.UNKNOWN;
}
case FUNCTION:
case CLASS:
case NEW:
case ARRAYLIT:
case OBJECTLIT:
return Tri.TRUE;
case VOID:
return Tri.FALSE;
case GETPROP:
case GETELEM:
case OPTCHAIN_GETELEM:
case OPTCHAIN_GETPROP:
// Assume that type information on getprops and getelems are likely to be wrong. This
// prevents spurious warnings from not including undefined in getelem's return value,
// from existence checks of symbols the externs define as certainly true, or from default
// initialization of globals ({@code x.y = x.y || {}}).
return Tri.UNKNOWN;
default:
}
// If we reach this point then all the composite structures that we can decompose have
// already been handled, leaving only qualified names and type-aware checks to handle below.
// Note that much of the switch above in fact duplicates the logic in getImpureBooleanValue,
// though with some subtle differences. Important differences include (1) avoiding recursion
// into the left-hand-side of nested logical operators, instead treating them as unknown since
// they would have already been reported elsewhere in the traversal had they been otherwise
// (this guarantees we visit each node once, rather than quadratically repeating work);
// (2) it propagates our unique amalgam of syntax-based and type-based checks to work when more
// deeply nested (i.e. recursively). These differences rely on assumptions that are very
// specific to this use case, so it does not make sense to upstream them.
Tri literalValue = NodeUtil.getBooleanValue(n);
if (literalValue != Tri.UNKNOWN || n.isName()) {
// If the truthiness is determinstic from the syntax then return that immediately.
// Alternatively, NAME nodes also get a pass since we don't trust the type information.
return literalValue;
}
JSType type = n.getJSType();
if (type != null) {
// Distrust types we think are always truthy, since sometimes the types lie, even for results
// of function calls (e.g. Map.prototype.get), so it's still important to check. But
// always-falsy values are a little more obviously wrong and there should be no reason for
// those type annotations to be lies. ANDing with UNKNOWN ensures we never return TRUE.
return Tri.UNKNOWN.and(type.getPossibleToBooleanOutcomes().toTri());
}
return Tri.UNKNOWN;
}
}