Index inside map() function

问题

I am missing a option how to get the index number inside the map function using List from Immutable.js:

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

Documentation shows that map() returns Iterable<number, M>. Is there any elegant way to what I need?

回答1:

You will be able to get the current iteration's index for the map method through its 2nd parameter.

Example:

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]
});

Output:

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

See also: 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.

回答2:

Array.prototype.map() index:

One can access the index Array.prototype.map() via the second argument of the callback function. Here is an example:

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


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

console.log(map);

Other arguments of Array.prototype.map():

• The third argument of the callback function exposes the array on which map was called upon
• The second argument of Array.map() is a object which will be the this value for the callback function. Keep in mind that you have to use the regular function keyword in order to declare the callback since an arrow function doesn't have its own binding to the this keyword.

For example:

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

const thisObj = {prop1: 1}


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

回答3:

Using Ramda:

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);

来源：https://stackoverflow.com/questions/38364400/index-inside-map-function