babel/src/babel/transformation/transformers/es6/destructuring.js

import * as messages from "../../../messages";
import * as t from "../../../types";

export var check = t.isPattern;

export function ForOfStatement(node, parent, scope, file) {
  var left = node.left;

  if (t.isPattern(left)) {
    // for ({ length: k } in { abc: 3 });

    var temp = scope.generateUidIdentifier("ref");

    node.left = t.variableDeclaration("var", [
      t.variableDeclarator(temp)
    ]);

    t.ensureBlock(node);

    node.body.body.unshift(t.variableDeclaration("var", [
      t.variableDeclarator(left, temp)
    ]));

    return;
  }

  if (!t.isVariableDeclaration(left)) return;

  var pattern = left.declarations[0].id;
  if (!t.isPattern(pattern)) return;

  var key = scope.generateUidIdentifier("ref");
  node.left = t.variableDeclaration(left.kind, [
    t.variableDeclarator(key, null)
  ]);

  var nodes = [];

  var destructuring = new DestructuringTransformer({
    kind: left.kind,
    file: file,
    scope: scope,
    nodes: nodes
  });

  destructuring.init(pattern, key);

  t.ensureBlock(node);

  var block = node.body;
  block.body = nodes.concat(block.body);
}

export { ForOfStatement as ForInStatement };

exports.Function = function (node, parent, scope, file) {
  var nodes = [];

  var hasDestructuring = false;

  node.params = node.params.map(function (pattern, i) {
    if (!t.isPattern(pattern)) return pattern;

    hasDestructuring = true;
    var ref = scope.generateUidIdentifier("ref");

    var destructuring = new DestructuringTransformer({
      blockHoist: node.params.length - i,
      nodes:      nodes,
      scope:      scope,
      file:       file,
      kind:       "let"
    });
    destructuring.init(pattern, ref);

    return ref;
  });

  if (!hasDestructuring) return;

  file.checkNode(nodes);
  t.ensureBlock(node);

  var block = node.body;
  block.body = nodes.concat(block.body);
};

export function CatchClause(node, parent, scope, file) {
  var pattern = node.param;
  if (!t.isPattern(pattern)) return;

  var ref = scope.generateUidIdentifier("ref");
  node.param = ref;

  var nodes = [];

  var destructuring = new DestructuringTransformer({
    kind: "let",
    file: file,
    scope: scope,
    nodes: nodes
  });
  destructuring.init(pattern, ref);

  node.body.body = nodes.concat(node.body.body);

  return node;
}

export function ExpressionStatement(node, parent, scope, file) {
  var expr = node.expression;
  if (expr.type !== "AssignmentExpression") return;
  if (!t.isPattern(expr.left)) return;
  if (file.isConsequenceExpressionStatement(node)) return;

  var destructuring = new DestructuringTransformer({
    operator: expr.operator,
    scope:    scope,
    file:     file,
  });

  return destructuring.init(expr.left, expr.right);
}

export function AssignmentExpression(node, parent, scope, file) {
  if (!t.isPattern(node.left)) return;

  var ref = scope.generateUidIdentifier("temp");

  var nodes = [];
  nodes.push(t.variableDeclaration("var", [
    t.variableDeclarator(ref, node.right)
  ]));

  var destructuring = new DestructuringTransformer({
    operator: node.operator,
    file: file,
    scope: scope,
    nodes: nodes
  });

  if (t.isArrayExpression(node.right)) {
    destructuring.arrays[ref.name] = true;
  }

  destructuring.init(node.left, ref);

  nodes.push(t.expressionStatement(ref));

  return nodes;
}

function variableDeclarationHasPattern(node) {
  for (var i = 0; i < node.declarations.length; i++) {
    if (t.isPattern(node.declarations[i].id)) {
      return true;
    }
  }
  return false;
}

export function VariableDeclaration(node, parent, scope, file) {
  if (t.isForInStatement(parent) || t.isForOfStatement(parent)) return;
  if (!variableDeclarationHasPattern(node)) return;

  var nodes = [];
  var declar;

  for (var i = 0; i < node.declarations.length; i++) {
    declar = node.declarations[i];

    var patternId = declar.init;
    var pattern   = declar.id;

    var destructuring = new DestructuringTransformer({
      nodes: nodes,
      scope: scope,
      kind:  node.kind,
      file:  file
    });

    if (t.isPattern(pattern) && patternId) {
      destructuring.init(pattern, patternId);

      if (+i !== node.declarations.length - 1) {
        // we aren't the last declarator so let's just make the
        // last transformed node inherit from us
        t.inherits(nodes[nodes.length - 1], declar);
      }
    } else {
      nodes.push(t.inherits(destructuring.buildVariableAssignment(declar.id, declar.init), declar));
    }
  }

  if (!t.isProgram(parent) && !t.isBlockStatement(parent)) {
    // https://github.com/babel/babel/issues/113
    // for (let [x] = [0]; false;) {}

    declar = null;

    for (i = 0; i < nodes.length; i++) {
      node = nodes[i];
      declar ||= t.variableDeclaration(node.kind, []);

      if (!t.isVariableDeclaration(node) && declar.kind !== node.kind) {
        throw file.errorWithNode(node, messages.get("invalidParentForThisNode"));
      }

      declar.declarations = declar.declarations.concat(node.declarations);
    }

    return declar;
  }

  return nodes;
}

function hasRest(pattern) {
  for (var i = 0; i < pattern.elements.length; i++) {
    if (t.isRestElement(pattern.elements[i])) {
      return true;
    }
  }
  return false;
}

var arrayUnpackVisitor = {
  enter(node, parent, scope, state) {
    if (this.isReferencedIdentifier() && state.bindings[node.name]) {
      state.deopt = true;
      this.stop();
    }
  }
};

class DestructuringTransformer {
  constructor(opts) {
    this.blockHoist = opts.blockHoist;
    this.operator   = opts.operator;
    this.arrays     = {};
    this.nodes      = opts.nodes || [];
    this.scope      = opts.scope;
    this.file       = opts.file;
    this.kind       = opts.kind;
  }

  buildVariableAssignment(id, init) {
    var op = this.operator;
    if (t.isMemberExpression(id)) op = "=";

    var node;

    if (op) {
      node = t.expressionStatement(t.assignmentExpression(op, id, init));
    } else {
      node = t.variableDeclaration(this.kind, [
        t.variableDeclarator(id, init)
      ]);
    }

    node._blockHoist = this.blockHoist;

    return node;
  }

  buildVariableDeclaration(id, init) {
    var declar = t.variableDeclaration("var", [
      t.variableDeclarator(id, init)
    ]);
    declar._blockHoist = this.blockHoist;
    return declar;
  }

  push(id, init) {
    if (t.isObjectPattern(id)) {
      this.pushObjectPattern(id, init);
    } else if (t.isArrayPattern(id)) {
      this.pushArrayPattern(id, init);
    } else if (t.isAssignmentPattern(id)) {
      this.pushAssignmentPattern(id, init);
    } else {
      this.nodes.push(this.buildVariableAssignment(id, init));
    }
  }

  toArray(node, count) {
    if (this.file.isLoose("es6.destructuring") || (t.isIdentifier(node) && this.arrays[node.name])) {
      return node;
    } else {
      return this.scope.toArray(node, count);
    }
  }

  pushAssignmentPattern(pattern, valueRef) {
    // we need to assign the current value of the assignment to avoid evaluating
    // it more than once

    var tempValueRef = this.scope.generateUidBasedOnNode(valueRef);

    var declar = t.variableDeclaration("var", [
      t.variableDeclarator(tempValueRef, valueRef)
    ]);
    declar._blockHoist = this.blockHoist;
    this.nodes.push(declar);

    //

    var tempConditional = t.conditionalExpression(
      t.binaryExpression("===", tempValueRef, t.identifier("undefined")),
      pattern.right,
      tempValueRef
    );

    var left = pattern.left;
    if (t.isPattern(left)) {
      this.nodes.push(t.expressionStatement(
        t.assignmentExpression("=", tempValueRef, tempConditional)
      ));
      this.push(left, tempValueRef);
    } else {
      this.nodes.push(this.buildVariableAssignment(left, tempConditional));
    }
  }

  pushObjectSpread(pattern, objRef, spreadProp, spreadPropIndex) {
    // get all the keys that appear in this object before the current spread

    var keys = [];

    for (var i = 0; i < pattern.properties.length; i++) {
      var prop = pattern.properties[i];

      // we've exceeded the index of the spread property to all properties to the
      // right need to be ignored
      if (i >= spreadPropIndex) break;

      // ignore other spread properties
      if (t.isSpreadProperty(prop)) continue;

      var key = prop.key;
      if (t.isIdentifier(key) && !prop.computed) key = t.literal(prop.key.name);
      keys.push(key);
    }

    keys = t.arrayExpression(keys);

    //

    var value = t.callExpression(this.file.addHelper("object-without-properties"), [objRef, keys]);
    this.nodes.push(this.buildVariableAssignment(spreadProp.argument, value));
  }

  pushObjectProperty(prop, propRef) {
    if (t.isLiteral(prop.key)) prop.computed = true;

    var pattern = prop.value;
    var objRef  = t.memberExpression(propRef, prop.key, prop.computed);

    if (t.isPattern(pattern)) {
      this.push(pattern, objRef);
    } else {
      this.nodes.push(this.buildVariableAssignment(pattern, objRef));
    }
  }

  pushObjectPattern(pattern, objRef) {
    // https://github.com/babel/babel/issues/681

    if (!pattern.properties.length) {
      this.nodes.push(t.expressionStatement(
        t.callExpression(this.file.addHelper("object-destructuring-empty"), [objRef])
      ));
    }

    // if we have more than one properties in this pattern and the objectRef is a
    // member expression then we need to assign it to a temporary variable so it's
    // only evaluated once

    if (pattern.properties.length > 1 && t.isMemberExpression(objRef)) {
      var temp = this.scope.generateUidBasedOnNode(objRef, this.file);
      this.nodes.push(this.buildVariableDeclaration(temp, objRef));
      objRef = temp;
    }

    //

    for (var i = 0; i < pattern.properties.length; i++) {
      var prop = pattern.properties[i];
      if (t.isSpreadProperty(prop)) {
        this.pushObjectSpread(pattern, objRef, prop, i);
      } else {
        this.pushObjectProperty(prop, objRef);
      }
    }
  }

  canUnpackArrayPattern(pattern, arr) {
    // not an array so there's no way we can deal with this
    if (!t.isArrayExpression(arr)) return false;

    // pattern has less elements than the array and doesn't have a rest so some
    // elements wont be evaluated
    if (pattern.elements.length > arr.elements.length) return;
    if (pattern.elements.length < arr.elements.length && !hasRest(pattern)) return false;

    for (var i = 0; i < pattern.elements.length; i++) {
      var elem = pattern.elements[i];

      // deopt on holes
      if (!elem) return false;

      // deopt on member expressions
      if (t.isMemberExpression(elem)) return false;
    }

    // deopt on reference to left side identifiers
    var bindings = t.getBindingIdentifiers(pattern);
    var state = { deopt: false, bindings };
    this.scope.traverse(arr, arrayUnpackVisitor, state);
    return !state.deopt;
  }

  pushUnpackedArrayPattern(pattern, arr) {
    for (var i = 0; i < pattern.elements.length; i++) {
      var elem = pattern.elements[i];
      if (t.isRestElement(elem)) {
        this.push(elem.argument, t.arrayExpression(arr.elements.slice(i)));
      } else {
        this.push(elem, arr.elements[i]);
      }
    }
  }

  pushArrayPattern(pattern, arrayRef) {
    if (!pattern.elements) return;

    // optimise basic array destructuring of an array expression
    //
    // we can't do this to a pattern of unequal size to it's right hand
    // array expression as then there will be values that wont be evaluated
    //
    // eg: var [a, b] = [1, 2];

    if (this.canUnpackArrayPattern(pattern, arrayRef)) {
      return this.pushUnpackedArrayPattern(pattern, arrayRef);
    }

    // if we have a rest then we need all the elements so don't tell
    // `scope.toArray` to only get a certain amount

    var count = !hasRest(pattern) && pattern.elements.length;

    // so we need to ensure that the `arrayRef` is an array, `scope.toArray` will
    // return a locally bound identifier if it's been inferred to be an array,
    // otherwise it'll be a call to a helper that will ensure it's one

    var toArray = this.toArray(arrayRef, count);

    if (t.isIdentifier(toArray)) {
      // we've been given an identifier so it must have been inferred to be an
      // array
      arrayRef = toArray;
    } else {
      arrayRef = this.scope.generateUidBasedOnNode(arrayRef);
      this.arrays[arrayRef.name] = true;
      this.nodes.push(this.buildVariableDeclaration(arrayRef, toArray));
    }

    //

    for (var i = 0; i < pattern.elements.length; i++) {
      var elem = pattern.elements[i];

      // hole
      if (!elem) continue;

      var elemRef;

      if (t.isRestElement(elem)) {
        elemRef = this.toArray(arrayRef);

        if (i > 0) {
          elemRef = t.callExpression(t.memberExpression(elemRef, t.identifier("slice")), [t.literal(i)]);
        }

        // set the element to the rest element argument since we've dealt with it
        // being a rest already
        elem = elem.argument;
      } else {
        elemRef = t.memberExpression(arrayRef, t.literal(i), true);
      }

      this.push(elem, elemRef);
    }
  }

  init(pattern, ref) {
    // trying to destructure a value that we can't evaluate more than once so we
    // need to save it to a variable

    var shouldMemoise = true;
    if (!t.isArrayExpression(ref) && !t.isMemberExpression(ref)) {
      var memo = this.scope.generateMemoisedReference(ref, true);
      if (memo) {
        this.nodes.push(this.buildVariableDeclaration(memo, ref));
        ref = memo;
      }
    }

    //

    this.push(pattern, ref);

    return this.nodes;
  }
}