Index inside map() function

Question

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?

Answer 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.