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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 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.jstype.JSType;
import java.util.HashSet;

/**
 * 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.error(
          "JSC_USE_OF_WITH", "The 'with' statement cannot be used in strict mode.");

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

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

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

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

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

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

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

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

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

  static final DiagnosticType DUPLICATE_MEMBER =
      DiagnosticType.warning(
          "JSC_DUPLICATE_MEMBER",
          "Class or object literal contains duplicate member \"{0}\". In non-strict code, the last"
              + " duplicate will overwrite the others.");

  private final AbstractCompiler compiler;
  private final CheckLevel defaultLevel;

  StrictModeCheck(AbstractCompiler compiler, CheckLevel defaultLevel) {
    this.compiler = compiler;
    this.defaultLevel = defaultLevel;
  }

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

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

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

  /**
   * 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 CATCH:
        return true;
      case FUNCTION:
        return n == n.getParent().getFirstChild();

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

      default:
        return false;
    }
  }

  /** Checks that an assignment is not to the "arguments" object. */
  private void checkAssignment(Node n) {
    if (n.getFirstChild().isName()) {
      if ("arguments".equals(n.getFirstChild().getString())) {
        this.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.
        this.report(n, EVAL_ASSIGNMENT);
      }
    }
  }

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

  /** Checks that object literal keys or class method names are valid. */
  private void checkObjectLiteralOrClass(Node n) {
    HashSet getters = new HashSet<>();
    HashSet setters = new HashSet<>();
    HashSet staticGetters = new HashSet<>();
    HashSet staticSetters = new HashSet<>();

    /*
     * Iterate backwards because the last duplicate is the one that will be used in sloppy or ES6
     * code. The earlier duplicates are the ones that should be removed.
     */
    for (Node key = n.getLastChild(); key != null; key = key.getPrevious()) {
      if (key.isEmpty()
          || key.isComputedProp()
          || key.isSpread()
          || key.isComputedFieldDef()
          // Computed properties cannot be computed at compile time
          || NodeUtil.isClassStaticBlock(key)
      // Will not check since whether duplicates are declared/assigned cannot be determined at
      // compile time since we do not know which code will be run
      ) {
        continue;
      }

      String keyName = key.getString();
      if (!key.isSetterDef()) {
        // normal property and getter cases
        HashSet set = key.isStaticMember() ? staticGetters : getters;
        if (!set.add(keyName)) {
          this.report(key, DUPLICATE_MEMBER, keyName);
        }
      }
      if (!key.isGetterDef()) {
        // normal property and setter cases
        HashSet set = key.isStaticMember() ? staticSetters : setters;
        if (!set.add(keyName)) {
          this.report(key, DUPLICATE_MEMBER, keyName);
        }
      }
    }
  }

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

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

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

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

  private void report(Node n, DiagnosticType diagnostic, String... args) {
    this.compiler.report(
        JSError.builder(diagnostic, args).setLevel(this.defaultLevel).setNode(n).build());
  }
}