es6-destructuring-visitors.js

/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */

/*global exports:true*/

/**
 * Implements ES6 destructuring assignment and pattern matchng.
 *
 * function init({port, ip, coords: [x, y]}) {
 *   return (x && y) ? {id, port} : {ip};
 * };
 *
 * function init($__0) {
 *   var
 *    port = $__0.port,
 *    ip = $__0.ip,
 *    $__1 = $__0.coords,
 *    x = $__1[0],
 *    y = $__1[1];
 *   return (x && y) ? {id, port} : {ip};
 * }
 *
 * var x, {ip, port} = init({ip, port});
 *
 * var x, $__0 = init({ip, port}), ip = $__0.ip, port = $__0.port;
 *
 */
var Syntax = require('esprima-fb').Syntax;
var utils = require('../src/utils');

var reservedWordsHelper = require('./reserved-words-helper');
var restParamVisitors = require('./es6-rest-param-visitors');
var restPropertyHelpers = require('./es7-rest-property-helpers');

// -------------------------------------------------------
// 1. Structured variable declarations.
//
// var [a, b] = [b, a];
// var {x, y} = {y, x};
// -------------------------------------------------------

function visitStructuredVariable(traverse, node, path, state) {
  // Allocate new temp for the pattern.
  utils.append(utils.getTempVar(state.localScope.tempVarIndex) + '=', state);
  // Skip the pattern and assign the init to the temp.
  utils.catchupWhiteSpace(node.init.range[0], state);
  traverse(node.init, path, state);
  utils.catchup(node.init.range[1], state);
  // Render the destructured data.
  utils.append(',' + getDestructuredComponents(node.id, state), state);
  state.localScope.tempVarIndex++;
  return false;
}

visitStructuredVariable.test = function(node, path, state) {
  return node.type === Syntax.VariableDeclarator &&
    isStructuredPattern(node.id);
};

function isStructuredPattern(node) {
  return node.type === Syntax.ObjectPattern ||
    node.type === Syntax.ArrayPattern;
}

// Main function which does actual recursive destructuring
// of nested complex structures.
function getDestructuredComponents(node, state) {
  var tmpIndex = state.localScope.tempVarIndex;
  var components = [];
  var patternItems = getPatternItems(node);

  for (var idx = 0; idx < patternItems.length; idx++) {
    var item = patternItems[idx];
    if (!item) {
      continue;
    }

    if (item.type === Syntax.SpreadElement) {
      // Spread/rest of an array.
      // TODO(dmitrys): support spread in the middle of a pattern
      // and also for function param patterns: [x, ...xs, y]
      components.push(item.argument.name +
        '=Array.prototype.slice.call(' +
        utils.getTempVar(tmpIndex) + ',' + idx + ')'
      );
      continue;
    }

    if (item.type === Syntax.SpreadProperty) {
      var restExpression = restPropertyHelpers.renderRestExpression(
        utils.getTempVar(tmpIndex),
        patternItems
      );
      components.push(item.argument.name + '=' + restExpression);
      continue;
    }

    // Depending on pattern type (Array or Object), we get
    // corresponding pattern item parts.
    var accessor = getPatternItemAccessor(node, item, tmpIndex, idx);
    var value = getPatternItemValue(node, item);

    // TODO(dmitrys): implement default values: {x, y=5}
    if (value.type === Syntax.Identifier) {
      // Simple pattern item.
      components.push(value.name + '=' + accessor);
    } else {
      // Complex sub-structure.
      components.push(
        utils.getTempVar(++state.localScope.tempVarIndex) + '=' + accessor +
        ',' + getDestructuredComponents(value, state)
      );
    }
  }

  return components.join(',');
}

function getPatternItems(node) {
  return node.properties || node.elements;
}

function getPatternItemAccessor(node, patternItem, tmpIndex, idx) {
  var tmpName = utils.getTempVar(tmpIndex);
  if (node.type === Syntax.ObjectPattern) {
    if (reservedWordsHelper.isReservedWord(patternItem.key.name)) {
      return tmpName + '["' + patternItem.key.name + '"]';
    } else if (patternItem.key.type === Syntax.Literal) {
      return tmpName + '[' + JSON.stringify(patternItem.key.value) + ']';
    } else if (patternItem.key.type === Syntax.Identifier) {
      return tmpName + '.' + patternItem.key.name;
    }
  } else if (node.type === Syntax.ArrayPattern) {
    return tmpName + '[' + idx + ']';
  }
}

function getPatternItemValue(node, patternItem) {
  return node.type === Syntax.ObjectPattern
    ? patternItem.value
    : patternItem;
}

// -------------------------------------------------------
// 2. Assignment expression.
//
// [a, b] = [b, a];
// ({x, y} = {y, x});
// -------------------------------------------------------

function visitStructuredAssignment(traverse, node, path, state) {
  var exprNode = node.expression;
  utils.append('var ' + utils.getTempVar(state.localScope.tempVarIndex) + '=', state);

  utils.catchupWhiteSpace(exprNode.right.range[0], state);
  traverse(exprNode.right, path, state);
  utils.catchup(exprNode.right.range[1], state);

  utils.append(
    ';' + getDestructuredComponents(exprNode.left, state) + ';',
    state
  );

  utils.catchupWhiteSpace(node.range[1], state);
  state.localScope.tempVarIndex++;
  return false;
}

visitStructuredAssignment.test = function(node, path, state) {
  // We consider the expression statement rather than just assignment
  // expression to cover case with object patters which should be
  // wrapped in grouping operator: ({x, y} = {y, x});
  return node.type === Syntax.ExpressionStatement &&
    node.expression.type === Syntax.AssignmentExpression &&
    isStructuredPattern(node.expression.left);
};

// -------------------------------------------------------
// 3. Structured parameter.
//
// function foo({x, y}) { ... }
// -------------------------------------------------------

function visitStructuredParameter(traverse, node, path, state) {
  utils.append(utils.getTempVar(getParamIndex(node, path)), state);
  utils.catchupWhiteSpace(node.range[1], state);
  return true;
}

function getParamIndex(paramNode, path) {
  var funcNode = path[0];
  var tmpIndex = 0;
  for (var k = 0; k < funcNode.params.length; k++) {
    var param = funcNode.params[k];
    if (param === paramNode) {
      break;
    }
    if (isStructuredPattern(param)) {
      tmpIndex++;
    }
  }
  return tmpIndex;
}

visitStructuredParameter.test = function(node, path, state) {
  return isStructuredPattern(node) && isFunctionNode(path[0]);
};

function isFunctionNode(node) {
  return (node.type == Syntax.FunctionDeclaration ||
    node.type == Syntax.FunctionExpression ||
    node.type == Syntax.MethodDefinition ||
    node.type == Syntax.ArrowFunctionExpression);
}

// -------------------------------------------------------
// 4. Function body for structured parameters.
//
// function foo({x, y}) { x; y; }
// -------------------------------------------------------

function visitFunctionBodyForStructuredParameter(traverse, node, path, state) {
  var funcNode = path[0];

  utils.catchup(funcNode.body.range[0] + 1, state);
  renderDestructuredComponents(funcNode, state);

  if (funcNode.rest) {
    utils.append(
      restParamVisitors.renderRestParamSetup(funcNode, state),
      state
    );
  }

  return true;
}

function renderDestructuredComponents(funcNode, state) {
  var destructuredComponents = [];

  for (var k = 0; k < funcNode.params.length; k++) {
    var param = funcNode.params[k];
    if (isStructuredPattern(param)) {
      destructuredComponents.push(
        getDestructuredComponents(param, state)
      );
      state.localScope.tempVarIndex++;
    }
  }

  if (destructuredComponents.length) {
    utils.append('var ' + destructuredComponents.join(',') + ';', state);
  }
}

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

exports.visitorList = [
  visitStructuredVariable,
  visitStructuredAssignment,
  visitStructuredParameter,
  visitFunctionBodyForStructuredParameter
];

exports.renderDestructuredComponents = renderDestructuredComponents;