您将能够通过其第二个参数index获取该map方法的当前迭代。

例子：

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

参数

callback - 生成新数组元素的函数，采用三个参数：

1) currentValue
数组中正在处理的当前元素。

2) index
当前正在处理的元素在数组中的索引。

3) array
调用了数组映射。

Array.prototype.map() 指数：

可以Array.prototype.map()通过回调函数的第二个参数访问索引。下面是一个例子：

const array = [1, 2, 3, 4];

const map = array.map((x, index) => {
  console.log(index);
  return x + index;
});

console.log(map);

的其他参数Array.prototype.map()：

• 回调函数的第三个参数公开了调用 map 的数组
• 的第二个参数Array.map()是一个对象，它将作为this回调函数的值。请记住，您必须使用常规function关键字来声明回调，因为箭头函数没有自己的this关键字绑定。

例如：

const array = [1, 2, 3, 4];

const thisObj = { prop1: 1 }

const map = array.map((x, index, array) => {
  console.log(array);
  console.log(this)
}, thisObj);

• 假设你有一个像

   const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
    
    
    arr.map((myArr, index) => {
      console.log(`your index is -> ${index} AND value is ${myArr}`);
    })

> output will be
 index is -> 0 AND value is 1
 index is -> 1 AND value is 2
 index is -> 2 AND value is 3
 index is -> 3 AND value is 4
 index is -> 4 AND value is 5
 index is -> 5 AND value is 6
 index is -> 6 AND value is 7
 index is -> 7 AND value is 8
 index is -> 8 AND value is 9

使用lambda：

import {addIndex, map} from 'ramda';

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