let-var 作用域链问题

@reverland

for of基本在任何平台对异步异常处理都不太对劲

怎么说?

若有所思... 此时的blockContext 应该与 前面的 tempContext 一样, 不重合~

看不懂

看了下原文说的Evaluation context部分, 应该是for循环每次都会创建, blockContext中的k 值每次都不一样, 就起到了跟var+自执行函数一样的效果

简单地说, 如果for括号里面有let或者const, 那么for的body每次执行的时候都会新建一个词法环境, 并且把let和const的声明的变量名和值绑定到这个新的词法环境, 所以有上面的结果.

不是很明白，for let应该给每个循环一个单独的作用域，虽然没看过标准，但感觉标准应该做符合直觉的事情。来填坑了无数人的坑。

另外，现在各种环境的es6支持称不上完美。for of基本在任何平台对异步异常处理都不太对劲。最好babel转下……

然而最近好像碰到了个babel的错误。。。

@William17

yeah, 类似用var加一个自执行函数每次都创建一个作用域...

我问JavaScript the core的作者这个问题有回复了 http://dmitrysoshnikov.com/ecmascript/javascript-the-core/comment-page-3/#comment-173985

Per ES6 let is a block-scoped. So every time we enter the block of the for loop, a new environment is created, and the function created inside the block, > is created relatively to this environment.

Roughly it’s similar to (although at engine level much more optimized than this):

var data = [];
for (var k = 0; k < 3; k++) {
  (function(x) {
    data[x] = function () {
      alert(x);
    };
  })(k);
}

@magicdawn 建一个函数的话，会进入一个新的执行上下文。 而含有词法声明的for只会新建一个词法环境对象。

@William17

一个是 Lexical Environment http://www.ecma-international.org/ecma-262/6.0/index.html#sec-lexical-environments 一个是 Execution Context http://www.ecma-international.org/ecma-262/6.0/index.html#sec-execution-contexts

Soga~

@magicdawn 不知道怎么回事。。。

@magicdawn 指这个例子，不知道表述有什么问题没。只是想说珍爱生命，慎用原生es6。

参见：https://hacks.mozilla.org/2015/07/es6-in-depth-generators-continued/

function setup() {
  console.log("setup")
}

function cleanup() {
  console.log("cleanup")
}

function work(value) {
  console.log(value)
}

function* produceValues() {
  setup();
  try {
    yield "work1";
    yield "work2";
    // ... yield some values ...
  } finally {
    cleanup();
  }
}

for (var value of produceValues()) {
  work(value);
  break;
}

这个例子，我这里node v5.0.0

➜  test  node forof.js    
setup
work1

firefox for ubuntu 43.0.4

setup
work1

babel转换后的

➜  test  cat forof-es5.js 

"use strict";
require("babel-polyfill");


var _marked = [produceValues].map(regeneratorRuntime.mark);

function setup() {
  console.log("setup");
}

function cleanup() {
  console.log("cleanup");
}

function work(value) {
  console.log(value);
}

function produceValues() {
  return regeneratorRuntime.wrap(function produceValues$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
          setup();
          _context.prev = 1;
          _context.next = 4;
          return "work1";

        case 4:
          _context.next = 6;
          return "work2";

        case 6:
          _context.prev = 6;

          cleanup();
          return _context.finish(6);

        case 9:
        case "end":
          return _context.stop();
      }
    }
  }, _marked[0], this, [[1,, 6, 9]]);
}

var _iteratorNormalCompletion = true;
var _didIteratorError = false;
var _iteratorError = undefined;

try {
  for (var _iterator = produceValues()[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
    var value = _step.value;

    work(value);
    break;
  }
} catch (err) {
  _didIteratorError = true;
  _iteratorError = err;
} finally {
  try {
    if (!_iteratorNormalCompletion && _iterator.return) {
      _iterator.return();
    }
  } finally {
    if (_didIteratorError) {
      throw _iteratorError;
    }
  }
}

➜  test  node forof-es5.js          
setup
work1
cleanup

@reverland

Note that the .return() is not called automatically by the language in all contexts, only in cases where the language uses the iteration protocol. So it is possible for a generator to be garbage collected without ever running its finally block.

这个不就是在用这个 iteration protocol么, 为啥没有调用 return? 发现node v4的generator没有return... 怎么回事

哈哈，不明觉厉...

@wangchi ......