es6-rest-param-visitors.js

/**
 * Copyright 2013 Facebook, Inc.
 *
 * 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.
 */

/*jslint node:true*/

/**
 * Desugars ES6 rest parameters into an ES3 arguments array.
 *
 * function printf(template, ...args) {
 *   args.forEach(...);
 * }
 *
 * We could use `Array.prototype.slice.call`, but that usage of arguments causes
 * functions to be deoptimized in V8, so instead we use a for-loop.
 *
 * function printf(template) {
 *   for (var args = [], $__0 = 1, $__1 = arguments.length; $__0 < $__1; $__0++)
 *     args.push(arguments[$__0]);
 *   args.forEach(...);
 * }
 *
 */
var Syntax = require('esprima-fb').Syntax;
var utils = require('../src/utils');



function _nodeIsFunctionWithRestParam(node) {
  return (node.type === Syntax.FunctionDeclaration
          || node.type === Syntax.FunctionExpression
          || node.type === Syntax.ArrowFunctionExpression)
         && node.rest;
}

function visitFunctionParamsWithRestParam(traverse, node, path, state) {
  if (node.parametricType) {
    utils.catchup(node.parametricType.range[0], state);
    path.unshift(node);
    traverse(node.parametricType, path, state);
    path.shift();
  }

  // Render params.
  if (node.params.length) {
    path.unshift(node);
    traverse(node.params, path, state);
    path.shift();
  } else {
    // -3 is for ... of the rest.
    utils.catchup(node.rest.range[0] - 3, state);
  }
  utils.catchupWhiteSpace(node.rest.range[1], state);

  path.unshift(node);
  traverse(node.body, path, state);
  path.shift();

  return false;
}

visitFunctionParamsWithRestParam.test = function(node, path, state) {
  return _nodeIsFunctionWithRestParam(node);
};

function renderRestParamSetup(functionNode, state) {
  var idx = state.localScope.tempVarIndex++;
  var len = state.localScope.tempVarIndex++;

  return 'for (var ' + functionNode.rest.name + '=[],' +
    utils.getTempVar(idx) + '=' + functionNode.params.length + ',' +
    utils.getTempVar(len) + '=arguments.length;' +
    utils.getTempVar(idx) + '<' +  utils.getTempVar(len) + ';' +
    utils.getTempVar(idx) + '++) ' +
    functionNode.rest.name + '.push(arguments[' + utils.getTempVar(idx) + ']);';
}

function visitFunctionBodyWithRestParam(traverse, node, path, state) {
  utils.catchup(node.range[0] + 1, state);
  var parentNode = path[0];
  utils.append(renderRestParamSetup(parentNode, state), state);
  return true;
}

visitFunctionBodyWithRestParam.test = function(node, path, state) {
  return node.type === Syntax.BlockStatement
         && _nodeIsFunctionWithRestParam(path[0]);
};

exports.renderRestParamSetup = renderRestParamSetup;
exports.visitorList = [
  visitFunctionParamsWithRestParam,
  visitFunctionBodyWithRestParam
];