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?
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:
- currentValue
The current element being processed in the array.2) index
The index of the current element being processed in the array.
- array
The array map was called upon.
map operation is use for constructing a new array by iterating through the elements of a given array. To answer your question, yes a return statement is required, and for this case it is returning the value 'X' on each iteration. Thus the final product of the code will be [ 'X', 'X','X','X' ]'X' is a string.
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():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);
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);
mapis supposed to preserve the structure of the array that is, only its values should be transformed not the array itself.