javascript - javascript 数组方法(map、forEach、reduce 等)的迭代顺序是否确定？

Question

使用一种 native 方法(map、forEach、reduce、filter 等)遍历数组的顺序是否确定并由标准保证？

EG，foo、bar、baz 和 qux 是否保证为 [0, 2, 6, 12]？

const a = [1, 2, 3, 4];
const foo = a.map((item, index) => item * index);
const bar = []; a.forEach((item, index) => bar[index] = item * index);
const baz = []; a.reduce((total, item, index) => baz[index] = item * index, 0);
const qux = []; a.filter((item, index) => qux[index] = item * index);
// etc

(这些是(非常)人为的例子)

Answer 1

为数组中存在的每个元素按升序调用回调函数。它不为缺少的元素调用。 (缺少元素？是的，JavaScript 处理稀疏数组)

var test = [];
test[30] = 'Test'; // sparse array, only one element defined.

test.forEach(
  function(value){
    console.log(value); // will only be called one time.
  }
);

来自标准:ECMA-262

22.1.3.10 Array.prototype.forEach ( callbackfn [ , thisArg ] )

NOTE 1

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

When the forEach method is called with one or two arguments, the following steps are taken:

1. Let O be ? ToObject(this value).
2. Let len be ? ToLength(? Get(O, "length")).
3. If IsCallable(callbackfn) is false, throw a TypeError exception.
4. If thisArg was supplied, let T be thisArg; else let T be undefined.
5. Let k be 0.
6. Repeat, while k < len a. Let Pk be ! ToString(k). b. Let kPresent be ? HasProperty(O, Pk). c. If kPresent is true, then i. Let kValue be ? Get(O, Pk). ii. Perform ? Call(callbackfn, T, « kValue, k, O »). d. Increase k by 1.
7. Return undefined.