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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 com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import javax.annotation.Nullable;

/**
 * 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 {

  private static final String CONST_FUNCTION_NAME = "goog.string.Const.from";
  private static final String CONST_FUNCTION_NAME_COLLAPSED = "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");

  @VisibleForTesting
  static final DiagnosticType CONST_NOT_ASSIGNED_STRING_LITERAL_ERROR =
      DiagnosticType.error("JSC_CONSTANT_NOT_ASSIGNED_STRING_LITERAL_ERROR",
          "Function argument is a variable {0} which is not a constant "
          + "assigned from a string literal");

  private final AbstractCompiler compiler;

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

  @Override
  public void process(Node externs, Node root) {
    Preconditions.checkState(compiler.getLifeCycleStage().isNormalized());
    NodeTraversal.traverseEs6(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)) {
        checkArgumentConstant(traversal, argument);
      }
    }
  }

  /**
   * Check the method call argument to be constant string literal.
   *
   * This function argument checker will yield an error if:
   * 

   * The argument is not a constant variable, or
   * 
The constant variable is not assigned from string literal or
   * concatenation thereof, or
   * 
The argument is not an expression that is a string literal or
   * concatenation thereof.
   * 
   *
   * @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 argument The node of function argument to check.
   */
  private void checkArgumentConstant(NodeTraversal traversal, Node argument) {
    if (argument.isName()) {
      String name = argument.getString();
      Scope scope = traversal.getScope();
      Var var = scope.getVar(name);
      if (var == null || !var.isInferredConst()) {
        compiler.report(traversal.makeError(
            argument, CONST_NOT_STRING_LITERAL_ERROR, name));
        return;
      }

      Node valueNode = var.getInitialValue();
      if (!isStringLiteralValue(valueNode)) {
        compiler.report(traversal.makeError(
            argument, CONST_NOT_ASSIGNED_STRING_LITERAL_ERROR, name));
      }
    } else {
      if (!isStringLiteralValue(argument)) {
        compiler.report(
            traversal.makeError(argument, CONST_NOT_STRING_LITERAL_ERROR));
      }
    }
  }

  /**
   * Returns true iff the value associated with the node is a JS string literal,
   * or a concatenation thereof.
   */
  private static boolean isStringLiteralValue(@Nullable Node node) {
    if (node == null) {
      return false;
    }
    if (node.isString()) {
      return true;
    } else if (node.getToken() == Token.ADD) {
      Preconditions.checkState(node.hasTwoChildren());
      Node left = node.getFirstChild();
      Node right = node.getLastChild();
      return isStringLiteralValue(left) && isStringLiteralValue(right);
    }
    return false;
  }
}