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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.javascript.jscomp.parsing.parser.FeatureSet;
import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;

/**
 * Splits variable declarations that declare multiple variables into separate declarations, if at
 * least one of the declarations is a destructuring declaration. For example
 *
 *  *   var [a, b] = foo(), c = bar();
 * 
 *
 * becomes
 *
 *  *   var [a, b] = foo();
 *   var c = bar();
 * 
 *
 * This runs before the main ES6 transpilation step, to simplify the transpilation of
 * destructuring syntax.
 */
public final class Es6SplitVariableDeclarations extends NodeTraversal.AbstractPostOrderCallback
    implements CompilerPass {
  private final AbstractCompiler compiler;
  private static final FeatureSet transpiledFeatures =
      FeatureSet.BARE_MINIMUM.with(Feature.ARRAY_DESTRUCTURING, Feature.OBJECT_DESTRUCTURING);

  public Es6SplitVariableDeclarations(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    TranspilationPasses.processTranspile(compiler, root, transpiledFeatures, this);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    if (NodeUtil.isDestructuringDeclaration(n)) {
      splitDeclaration(t, n, parent);
    }
  }

  public void splitDeclaration(NodeTraversal t, Node n, Node parent) {
    // Cannot split cases like "for (let a = 3, [b] = arr; ..." or "a: let x = 3, [y] = arr;" yet
    // that are not in a statement block.
    if (n.hasMoreThanOneChild()) {
      if (NodeUtil.isStatementBlock(parent)) {
        // These are the obvious cases.
      } else if (parent.isVanillaFor()) {
        // For-loop initializers are handled specially in Es6RewriteDestructuring
        return;
      } else {
        t.report(
            n,
            TranspilationUtil.CANNOT_CONVERT_YET,
            "declaration with multiple destructuring children not in statement block");
        return;
      }
    }

    while (n.getFirstChild() != n.getLastChild()) {
      Node child = n.getLastChild().detach();
      Node newVar = IR.declaration(child, n.getToken()).srcref(n);
      newVar.insertAfter(n);
      t.reportCodeChange();
    }
  }
}