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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 2013 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.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.VisibleForTesting;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/**
 * Enforces that invocations of the method {@code goog.string.Const.from} are done with an argument
 * which is a string literal.
 *
 * This function parameter checker enforces that for all invocations of method {@code
 * goog.string.Const.from} the actual argument satisfies one of the following conditions:
 *
 * 

 *   The argument is an expression that is a string literal or concatenation thereof, or
 *   
The argument is a constant variable previously assigned from a string literal or
 *       concatenation thereof.
 * 
 */
class ConstParamCheck extends AbstractPostOrderCallback implements CompilerPass {

  // NOTE: Comparison of Node qualified names is faster than comparing against strings.
  private static final Node CONST_FUNCTION_NAME =
      IR.getprop(IR.name("goog"), "string", "Const", "from");
  private static final Node CONST_FUNCTION_NAME_COLLAPSED = IR.name("goog$string$Const$from");

  @VisibleForTesting
  static final DiagnosticType CONST_NOT_STRING_LITERAL_ERROR =
      DiagnosticType.error(
          "JSC_CONSTANT_NOT_STRING_LITERAL_ERROR",
          "Function argument is not a string literal or a constant assigned "
              + "from a string literal or a concatenation of these.");

  private final AbstractCompiler compiler;

  public ConstParamCheck(AbstractCompiler compiler) {
    this.compiler = checkNotNull(compiler);
  }

  @Override
  public void process(Node externs, Node root) {
    checkState(compiler.getLifeCycleStage().isNormalized());
    NodeTraversal.traverse(compiler, root, this);
  }

  /**
   * Callback to visit a node and check method call arguments of {@code goog.string.Const.from}.
   *
   * @param traversal The node traversal object that supplies context, such as the scope chain to
   *     use in name lookups as well as error reporting.
   * @param node The node being visited.
   * @param parent The parent of the node.
   */
  @Override
  public void visit(NodeTraversal traversal, Node node, Node parent) {
    if (node.isCall()) {
      Node name = node.getFirstChild();
      Node argument = name.getNext();
      if (argument == null) {
        return;
      }

      // Detect calls to an aliased goog.string.Const.
      if (name.isName() && !name.matchesQualifiedName(CONST_FUNCTION_NAME_COLLAPSED)) {
        Scope scope = traversal.getScope();
        Var var = scope.getVar(name.getString());
        if (var == null) {
          return;
        }
        name = var.getInitialValue();
        if (name == null) {
          return;
        }
      }

      if (name.matchesQualifiedName(CONST_FUNCTION_NAME)
          || name.matchesQualifiedName(CONST_FUNCTION_NAME_COLLAPSED)) {
        if (!isSafeValue(traversal.getScope(), argument)) {
          compiler.report(JSError.make(argument, CONST_NOT_STRING_LITERAL_ERROR));
        }
      }
    }
  }

  /**
   * Checks if the method call argument is made of constant string literals.
   *
   * This function argument checker will return true if:
   *
   * 

   *   The argument is a constant variable assigned from a string literal, or
   *   
The argument is a template into which only string literals are inserted, or
   *   
The argument is an expression that is a string literal, or
   *   
The argument is a ternary expression choosing between string literals, or
   *   
The argument is a concatenation of the above.
   * 
   *
   * @param scope The scope chain to use in name lookups.
   * @param argument The node of function argument to check.
   */
  private boolean isSafeValue(Scope scope, Node argument) {
    if (NodeUtil.isSomeCompileTimeConstStringValue(argument)) {
      return true;
    } else if (argument.isTemplateLit()) {
      // Each templateLit child is either a TemplateLitString, or has children which are substituted
      for (Node sub = argument.getFirstChild(); sub != null; sub = sub.getNext()) {
        if (sub.isTemplateLitString()) {
          continue;
        }
        if (!isSafeValue(scope, sub.getOnlyChild())) {
          return false;
        }
      }
      return true;
    } else if (argument.isAdd()) {
      Node left = argument.getFirstChild();
      Node right = argument.getLastChild();
      return isSafeValue(scope, left) && isSafeValue(scope, right);
    } else if (argument.isName()) {
      String name = argument.getString();
      Var var = scope.getVar(name);
      if (var == null || !var.isDeclaredOrInferredConst()) {
        return false;
      }
      Node initialValue = var.getInitialValue();
      if (initialValue == null) {
        return false;
      }
      return isSafeValue(var.getScope(), initialValue);
    }
    return false;
  }
}