Index inside map() function
JavascriptFunctional Programmingimmutable.jsJavascript Problem Overview
I am missing a option how to get the index number inside the map
function using List
from Immutable.js
:
Documentation shows that map()
returns Iterable<number, M>
. Is there any elegant way to what I need?
Javascript Solutions
Solution 1 - Javascript
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.
Solution 2 - Javascript
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);
Array.prototype.map()
:
Other arguments of - 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 thethis
value for the callback function. Keep in mind that you have to use the regularfunction
keyword in order to declare the callback since an arrow function doesn't have its own binding to thethis
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);
Solution 3 - Javascript
-
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
Solution 4 - Javascript
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);