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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?

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?

• javascript
• functional-programming
• immutable.js

Share Improve this question Follow asked Jul 14, 2016 at 2:07 ZygimantasZygimantas 8,73788 gold badges4343 silver badges5555 bronze badges 2

• 1 Keep in mind that map is supposed to preserve the structure of the array that is, only its values should be transformed not the array itself. – user6445533 Commented Jul 14, 2016 at 6:12
• Array.prototype.map() create a new array using the callback function as transformation – Carmine Tambascia Commented Oct 15, 2021 at 20:18

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4 Answers 4

Sorted by: Reset to default 1062

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.

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

• suppose you have an array like

   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

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);

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