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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.
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/*
 * Copyright 2009 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.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.TypeI;

import java.util.HashSet;
import java.util.Set;

/**
 * Checks that the code obeys the static restrictions of strict mode:
 * 
 *  No use of "with".
 * 
 No deleting variables, functions, or arguments.
 * 
 No re-declarations or assignments of "eval" or arguments.
 * 
 No use of arguments.callee
 * 
 No use of arguments.caller
 * 
 Class: Always under strict mode
 * 
   In addition, no duplicate class method names
 * 
 *
 */
class StrictModeCheck extends AbstractPostOrderCallback
    implements CompilerPass {

  static final DiagnosticType USE_OF_WITH = DiagnosticType.warning(
      "JSC_USE_OF_WITH",
      "The 'with' statement cannot be used in ES5 strict mode.");

  static final DiagnosticType EVAL_DECLARATION = DiagnosticType.warning(
      "JSC_EVAL_DECLARATION",
      "\"eval\" cannot be redeclared in ES5 strict mode");

  static final DiagnosticType EVAL_ASSIGNMENT = DiagnosticType.warning(
      "JSC_EVAL_ASSIGNMENT",
      "the \"eval\" object cannot be reassigned in ES5 strict mode");

  static final DiagnosticType ARGUMENTS_DECLARATION = DiagnosticType.warning(
      "JSC_ARGUMENTS_DECLARATION",
      "\"arguments\" cannot be redeclared in ES5 strict mode");

  static final DiagnosticType ARGUMENTS_ASSIGNMENT = DiagnosticType.warning(
      "JSC_ARGUMENTS_ASSIGNMENT",
      "the \"arguments\" object cannot be reassigned in ES5 strict mode");

  static final DiagnosticType ARGUMENTS_CALLEE_FORBIDDEN = DiagnosticType.warning(
      "JSC_ARGUMENTS_CALLEE_FORBIDDEN",
      "\"arguments.callee\" cannot be used in ES5 strict mode");

  static final DiagnosticType ARGUMENTS_CALLER_FORBIDDEN = DiagnosticType.warning(
      "JSC_ARGUMENTS_CALLER_FORBIDDEN",
      "\"arguments.caller\" cannot be used in ES5 strict mode");

  static final DiagnosticType FUNCTION_CALLER_FORBIDDEN = DiagnosticType.warning(
      "JSC_FUNCTION_CALLER_FORBIDDEN",
      "A function''s \"caller\" property cannot be used in ES5 strict mode");

  static final DiagnosticType FUNCTION_ARGUMENTS_PROP_FORBIDDEN = DiagnosticType.warning(
      "JSC_FUNCTION_ARGUMENTS_PROP_FORBIDDEN",
      "A function''s \"arguments\" property cannot be used in ES5 strict mode");

  static final DiagnosticType DELETE_VARIABLE = DiagnosticType.warning(
      "JSC_DELETE_VARIABLE",
      "variables, functions, and arguments cannot be deleted in ES5 strict mode");

  static final DiagnosticType DUPLICATE_OBJECT_KEY = DiagnosticType.error(
      "JSC_DUPLICATE_OBJECT_KEY",
      "Object literal contains illegal duplicate key \"{0}\", disallowed in ES5 strict mode");

  static final DiagnosticType DUPLICATE_CLASS_METHODS = DiagnosticType.error(
      "JSC_DUPLICATE_CLASS_METHODS",
      "Class contain duplicate method name \"{0}\"");

  static final DiagnosticType BAD_FUNCTION_DECLARATION = DiagnosticType.error(
      "JSC_BAD_FUNCTION_DECLARATION",
      "functions can only be declared at top level or immediately within"
      + " another function in ES5 strict mode");

  private final AbstractCompiler compiler;

  StrictModeCheck(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override public void process(Node externs, Node root) {
    NodeTraversal.traverseRootsEs6(compiler, this, externs, root);
    NodeTraversal.traverseEs6(compiler, root, new NonExternChecks());
  }

  @Override public void visit(NodeTraversal t, Node n, Node parent) {
    if (n.isFunction()) {
      checkFunctionUse(t, n);
    } else if (n.isAssign()) {
      checkAssignment(t, n);
    } else if (n.isDelProp()) {
      checkDelete(t, n);
    } else if (n.isObjectLit()) {
      checkObjectLiteralOrClass(t, n);
    } else if (n.isClass()) {
      checkObjectLiteralOrClass(t, n.getLastChild());
    } else if (n.isWith()) {
      checkWith(t, n);
    }
  }

  /** Reports a warning for with statements. */
  private static void checkWith(NodeTraversal t, Node n) {
    JSDocInfo info = n.getJSDocInfo();
    boolean allowWith =
        info != null && info.getSuppressions().contains("with");
    if (!allowWith) {
      t.report(n, USE_OF_WITH);
    }
  }

  /** Checks that the function is used legally. */
  private static void checkFunctionUse(NodeTraversal t, Node n) {
    if (NodeUtil.isFunctionDeclaration(n) && !NodeUtil.isHoistedFunctionDeclaration(n)) {
      t.report(n, BAD_FUNCTION_DECLARATION);
    }
  }

  /**
   * Determines if the given name is a declaration, which can be a declaration
   * of a variable, function, or argument.
   */
  private static boolean isDeclaration(Node n) {
    switch (n.getParent().getToken()) {
      case LET:
      case CONST:
      case VAR:
      case FUNCTION:
      case CATCH:
        return true;

      case PARAM_LIST:
        return n.getGrandparent().isFunction();

      default:
        return false;
    }
  }

  /** Checks that an assignment is not to the "arguments" object. */
  private static void checkAssignment(NodeTraversal t, Node n) {
    if (n.getFirstChild().isName()) {
      if ("arguments".equals(n.getFirstChild().getString())) {
        t.report(n, ARGUMENTS_ASSIGNMENT);
      } else if ("eval".equals(n.getFirstChild().getString())) {
        // Note that assignment to eval is already illegal because any use of
        // that name is illegal.
        t.report(n, EVAL_ASSIGNMENT);
      }
    }
  }

  /** Checks that variables, functions, and arguments are not deleted. */
  private static void checkDelete(NodeTraversal t, Node n) {
    if (n.getFirstChild().isName()) {
      Var v = t.getScope().getVar(n.getFirstChild().getString());
      if (v != null) {
        t.report(n, DELETE_VARIABLE);
      }
    }
  }

  /** Checks that object literal keys or class method names are valid. */
  private static void checkObjectLiteralOrClass(NodeTraversal t, Node n) {
    Set getters = new HashSet<>();
    Set setters = new HashSet<>();
    for (Node key = n.getFirstChild();
         key != null;
         key = key.getNext()) {
      String keyName = key.getString();
      if (!key.isSetterDef()) {
        // normal property and getter cases
        if (!getters.add(keyName)) {
          if (n.isClassMembers()) {
            t.report(key, DUPLICATE_CLASS_METHODS, keyName);
          } else {
            t.report(key, DUPLICATE_OBJECT_KEY, keyName);
          }
        }
      }
      if (!key.isGetterDef()) {
        // normal property and setter cases
        if (!setters.add(keyName)) {
          if (n.isClassMembers()) {
            t.report(key, DUPLICATE_CLASS_METHODS, keyName);
          } else {
            t.report(key, DUPLICATE_OBJECT_KEY, keyName);
          }
        }
      }
    }
  }

  /** Checks that are performed on non-extern code only. */
  private static class NonExternChecks extends AbstractPostOrderCallback {
    @Override public void visit(NodeTraversal t, Node n, Node parent) {
      if ((n.isName()) && isDeclaration(n)) {
        checkDeclaration(t, n);
      } else if (n.isGetProp()) {
        checkGetProp(t, n);
      }
    }

    /** Checks for illegal declarations. */
    private void checkDeclaration(NodeTraversal t, Node n) {
      if ("eval".equals(n.getString())) {
        t.report(n, EVAL_DECLARATION);
      } else if ("arguments".equals(n.getString())) {
        t.report(n, ARGUMENTS_DECLARATION);
      }
    }

    /** Checks that the arguments.callee is not used. */
    private void checkGetProp(NodeTraversal t, Node n) {
      Node target = n.getFirstChild();
      Node prop = n.getLastChild();
      if (prop.getString().equals("callee")) {
        if (target.isName() && target.getString().equals("arguments")) {
          t.report(n, ARGUMENTS_CALLEE_FORBIDDEN);
        }
      } else if (prop.getString().equals("caller")) {
        if (target.isName() && target.getString().equals("arguments")) {
          t.report(n, ARGUMENTS_CALLER_FORBIDDEN);
        } else if (isFunctionType(target)) {
          t.report(n, FUNCTION_CALLER_FORBIDDEN);
        }
      } else if (prop.getString().equals("arguments")
          && isFunctionType(target)) {
        t.report(n, FUNCTION_ARGUMENTS_PROP_FORBIDDEN);
      }
    }
  }

  private static boolean isFunctionType(Node n) {
    TypeI type = n.getTypeI();
    return (type != null && type.isFunctionType());
  }
}