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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 2007 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.rhino.JSDocInfo;
import com.google.javascript.rhino.JSTypeExpression;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import org.jspecify.nullness.Nullable;

/**
 * Checks for certain uses of the {@code this} keyword that are considered unsafe because they are
 * likely to reference the global {@code this} object unintentionally.
 *
 * A use of {@code this} is considered unsafe if it's on the left side of an assignment or a
 * property access, and not inside one of the following:
 *
 * 

 *   a class static initialization block
 *   
a prototype method
 *   
a function annotated with {@code @constructor}
 *   
a function annotated with {@code @this}.
 *   
a function where there's no logical place to put a {@code this} annotation.
 * 
 *
 * Note that this check does not track assignments of {@code this} to variables or objects. The
 * code
 *
 * 
 * function evil() {
 *   var a = this;
 *   a.useful = undefined;
 * }
 * 
 *
 * will not get flagged, even though it is semantically equivalent to
 *
 *  * function evil() {
 *   this.useful = undefined;
 * }
 * 
 *
 * which would get flagged.
 */
final class CheckGlobalThis implements NodeTraversal.Callback {

  static final DiagnosticType GLOBAL_THIS = DiagnosticType.warning(
      "JSC_USED_GLOBAL_THIS",
      "dangerous use of the global 'this' object");

  private final AbstractCompiler compiler;

  /**
   * If {@code assignLhsChild != null}, then the node being traversed is a descendant of the first
   * child of an ASSIGN node. assignLhsChild's parent is this ASSIGN node.
   */
  private @Nullable Node assignLhsChild = null;

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

  /**
   * Since this pass reports errors only when a global {@code this} keyword
   * is encountered, there is no reason to traverse non global contexts.
   */
  @Override
  public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {

    if (n.isFunction()) {
      // Arrow functions automatically get the "this" value from the
      // enclosing scope. e.g. the "this" in
      //   Foo.prototype.getBar = () => this.bar;
      // is the global "this", not an instance of Foo.
      if (n.isArrowFunction()) {
        return true;
      }

      // Don't traverse functions that are constructors or have the @this
      // or @override annotation.
      JSDocInfo jsDoc = NodeUtil.getBestJSDocInfo(n);
      if (jsDoc != null &&
          (jsDoc.isConstructor() ||
           jsDoc.isInterface() ||
           jsDoc.hasThisType() ||
           jsDoc.isOverride())) {
        return false;
      }

      if (jsDoc != null) {
        JSTypeExpression functionType = jsDoc.getType();
        if (functionType != null) {
          Node functionNode = functionType.getRoot();
          if (functionNode != null && functionNode.isFunction()) {
            // function(this: ThisType, ...)
            // `this:` is only allowed as the very first child of the
            // FUNCTION node.
            Node thisNode = functionNode.getFirstChild();
            if (thisNode != null && thisNode.isThis()) {
              // Type of `this` is specified, so no need to check further.
              return false;
            }
          }
        }
      }

      // Don't traverse functions unless they would normally
      // be able to have a @this annotation associated with them. e.g.,
      // var a = function() { }; // or
      // function a() {} // or
      // a.x = function() {}; // or
      // var a = {x: function() {}};
      Token pType = parent.getToken();
      if (!(pType == Token.BLOCK ||
            pType == Token.SCRIPT ||
            pType == Token.NAME ||
            pType == Token.ASSIGN ||

            // object literal keys
            pType == Token.STRING_KEY)) {
        return false;
      }

      // Don't traverse functions that are getting lent to a prototype.
      Node grandparent = parent.getParent();
      if (NodeUtil.mayBeObjectLitKey(parent)) {
        JSDocInfo maybeLends = grandparent.getJSDocInfo();
        if (maybeLends != null
            && maybeLends.hasLendsName()
            && maybeLends.getLendsName().getRoot().getString().endsWith(".prototype")) {
          return false;
        }
      }
    }

    // Don't traverse class static blocks, 'this' is never the global 'this'
    if (NodeUtil.isClassStaticBlock(n)) {
      return false;
    }

    if (parent != null && parent.isAssign()) {
      Node lhs = parent.getFirstChild();

      if (n == lhs) {
        // Always traverse the left side of the assignment. To handle
        // nested assignments properly (e.g., (a = this).property = c;),
        // assignLhsChild should not be overridden.
        if (assignLhsChild == null) {
          assignLhsChild = lhs;
        }
      } else {
        // Only traverse the right side if it's not an assignment to a prototype
        // property or subproperty.
        if (NodeUtil.isNormalGet(lhs)) {
          if (lhs.isGetProp() && lhs.getString().equals("prototype")) {
            return false;
          }
          Node llhs = lhs.getFirstChild();
          if (llhs.isGetProp() && llhs.getString().equals("prototype")) {
            return false;
          }
        }
      }
    }

    return true;
  }

  @Override
  public void visit(NodeTraversal unused, Node n, Node parent) {
    if (n.isThis() && shouldReportThis(n)) {
      compiler.report(JSError.make(n, GLOBAL_THIS));
    }
    if (n == assignLhsChild) {
      assignLhsChild = null;
    }
  }

  private boolean shouldReportThis(Node n) {
    Node parent = n.getParent();
    if (assignLhsChild != null) {
      // Always report a THIS on the left side of an assign.
      return true;
    }

    // Also report a THIS with a property access.
    return parent != null && NodeUtil.isNormalGet(parent);
  }
}