javascript - map() 函数内的索引

Question

我错过了如何获取 map 中的索引号的选项函数使用 List来自 Immutable.js :

var list2 = list1.map(mapper => { a: mapper.a, b: mapper.index??? }).toList();

Documentation shows那map()返回 Iterable<number, M> .有什么优雅的方法可以满足我的需要吗？

Answer 1

您将能够通过其第二个参数为 map 方法获取当前迭代的 index。

示例:

const list = [ 'h', 'e', 'l', 'l', 'o'];
list.map((currElement, index) => {
  console.log("The current iteration is: " + index);
  console.log("The current element is: " + currElement);
  console.log("\n");
  return currElement; //equivalent to list[index]
});

输出:

The current iteration is: 0 <br>The current element is: h

The current iteration is: 1 <br>The current element is: e

The current iteration is: 2 <br>The current element is: l

The current iteration is: 3 <br>The current element is: l 

The current iteration is: 4 <br>The current element is: o

另见: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/map

Parameters

callback - Function that produces an element of the new Array, taking three arguments:

1) currentValue
The current element being processed in the array.

2) index
The index of the current element being processed in the array.

3) array
The array map was called upon.