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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 2014 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.base.Preconditions;
import com.google.javascript.jscomp.SyntacticScopeCreator.DefaultRedeclarationHandler;
import com.google.javascript.jscomp.SyntacticScopeCreator.RedeclarationHandler;
import com.google.javascript.rhino.InputId;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/**
 * The syntactic scope creator scans the parse tree to create a Scope object
 * containing all the variable declarations in that scope. This class adds supported
 * for block-level scopes introduced in ECMAScript 6.
 *
 * This implementation is not thread-safe.
 *
 * @author [email protected] (Michael Zhou)
 */
class Es6SyntacticScopeCreator implements ScopeCreator {
  private final AbstractCompiler compiler;
  private Scope scope;
  private InputId inputId;
  private final RedeclarationHandler redeclarationHandler;

  // The arguments variable is special, in that it's declared for every function,
  // but not explicitly declared.
  private static final String ARGUMENTS = "arguments";

  Es6SyntacticScopeCreator(AbstractCompiler compiler) {
    this.compiler = compiler;
    this.redeclarationHandler = new DefaultRedeclarationHandler();
  }

  Es6SyntacticScopeCreator(
      AbstractCompiler compiler, RedeclarationHandler redeclarationHandler) {
    this.compiler = compiler;
    this.redeclarationHandler = redeclarationHandler;
  }

  @Override
  public Scope createScope(Node n, Scope parent) {
    inputId = null;
    if (parent == null) {
      scope = Scope.createGlobalScope(n);
    } else {
      scope = new Scope(parent, n);
    }

    scanRoot(n);

    inputId = null;
    Scope returnedScope = scope;
    scope = null;
    return returnedScope;
  }

  private void scanRoot(Node n) {
    if (n.isFunction()) {
      if (inputId == null) {
        inputId = NodeUtil.getInputId(n);
        // TODO(johnlenz): inputId maybe null if the FUNCTION node is detached
        // from the AST.
        // Is it meaningful to build a scope for detached FUNCTION node?
      }

      final Node fnNameNode = n.getFirstChild();
      final Node args = fnNameNode.getNext();

      // Bleed the function name into the scope, if it hasn't
      // been declared in the outer scope.
      String fnName = fnNameNode.getString();
      if (!fnName.isEmpty() && NodeUtil.isFunctionExpression(n)) {
        declareVar(fnNameNode);
      }

      // Args: Declare function variables
      Preconditions.checkState(args.isParamList());
      declareLHS(scope, args);
      // Since we create a separate scope for body, stop scanning here

    } else if (n.isClass()) {
      if (scope.getParent() != null) {
        inputId = NodeUtil.getInputId(n);
      }

      final Node classNameNode = n.getFirstChild();
      // Bleed the class name into the scope, if it hasn't
      // been declared in the outer scope.
      if (!classNameNode.isEmpty()) {
        if (NodeUtil.isClassExpression(n)) {
          declareVar(classNameNode);
        }
      }
    } else if (n.isBlock() || n.isFor() || n.isForOf() || n.isSwitch()) {
      if (scope.getParent() != null) {
        inputId = NodeUtil.getInputId(n);
      }
      scanVars(n);
    } else {
      // n is the global SCRIPT node
      Preconditions.checkState(scope.getParent() == null);
      scanVars(n);
    }
  }

  private void declareLHS(Scope declarationScope, Node n) {
    for (Node lhs : NodeUtil.getLhsNodesOfDeclaration(n)) {
      declareVar(declarationScope, lhs);
    }
  }

  /**
    * Scans and gather variables declarations under a Node
    */
  private void scanVars(Node n) {
    switch (n.getType()) {
      case Token.VAR:
        declareLHS(scope.getClosestHoistScope(), n);
        return;

      case Token.LET:
      case Token.CONST:
        // Only declare when scope is the current lexical scope
        if (!isNodeAtCurrentLexicalScope(n)) {
          return;
        }
        declareLHS(scope, n);
        return;

      case Token.FUNCTION:
        if (NodeUtil.isFunctionExpression(n) || !isNodeAtCurrentLexicalScope(n)) {
          return;
        }

        String fnName = n.getFirstChild().getString();
        if (fnName.isEmpty()) {
          // This is invalid, but allow it so the checks can catch it.
          return;
        }
        declareVar(n.getFirstChild());
        return;   // should not examine function's children

      case Token.CLASS:
        if (NodeUtil.isClassExpression(n) || !isNodeAtCurrentLexicalScope(n)) {
          return;
        }
        String className = n.getFirstChild().getString();
        if (className.isEmpty()) {
          // This is invalid, but allow it so the checks can catch it.
          return;
        }
        declareVar(n.getFirstChild());
        return;  // should not examine class's children

      case Token.CATCH:
        Preconditions.checkState(n.getChildCount() == 2, n);
        // the first child is the catch var and the second child
        // is the code block
        if (isNodeAtCurrentLexicalScope(n)) {
          declareLHS(scope, n);
        }
        // A new scope is not created for this BLOCK because there is a scope
        // created for the BLOCK above the CATCH
        final Node block = n.getSecondChild();
        scanVars(block);
        return;  // only one child to scan

      case Token.SCRIPT:
        inputId = n.getInputId();
        Preconditions.checkNotNull(inputId);
        break;
    }

    // Variables can only occur in statement-level nodes, so
    // we only need to traverse children in a couple special cases.
    if (NodeUtil.isControlStructure(n) || NodeUtil.isStatementBlock(n)) {
      for (Node child = n.getFirstChild();
           child != null;) {
        Node next = child.getNext();
        scanVars(child);
        child = next;
      }
    }
  }

  private void declareVar(Node n) {
    declareVar(scope, n);
  }

  /**
   * Declares a variable.
   *
   * @param s The scope to declare the variable in.
   * @param n The node corresponding to the variable name.
   */
  private void declareVar(Scope s, Node n) {
    Preconditions.checkState(n.isName() || n.isStringKey(),
        "Invalid node for declareVar: %s", n);

    String name = n.getString();
    // Because of how we scan the variables, it is possible to encounter
    // the same var declared name node twice. Bail out in this case.
    if (s.getVar(name) != null && s.getVar(name).getNode() == n) {
      return;
    }

    CompilerInput input = compiler.getInput(inputId);
    if (s.isDeclared(name, false) || (s.isLocal() && name.equals(ARGUMENTS))) {
      redeclarationHandler.onRedeclaration(s, name, n, input);
    } else {
      s.declare(name, n, input);
    }
  }

  /**
   * Determines whether the name should be declared at current lexical scope.
   * Assume the parent node is a BLOCK, FOR, FOR_OF, SCRIPT or LABEL.
   *
   * @param n The declaration node to be checked
   * @return whether the name should be declared at current lexical scope
   */
  private boolean isNodeAtCurrentLexicalScope(Node n) {
    Node parent = n.getParent();
    Node grandparent = parent.getParent();
    Preconditions.checkState(parent.isBlock() || parent.isFor() || parent.isForOf()
        || parent.isScript() || parent.isLabel(), parent);

    if (parent.isSyntheticBlock()
        && grandparent != null && (grandparent.isCase() || grandparent.isDefaultCase())) {
      Node switchNode = grandparent.getParent();
      return scope.getRootNode() == switchNode;
    }

    if (parent == scope.getRootNode() || parent.isScript()
        || (grandparent.isCatch()
            && parent.getGrandparent() == scope.getRootNode())) {
      return true;
    }

    while (parent.isLabel()) {
      if (parent.getParent() == scope.getRootNode()) {
        return true;
      }
      parent = parent.getParent();
    }
    return false;
  }

  @Override
  public boolean hasBlockScope() {
    return true;
  }
}