Javascript - Map function (Functional programming)


The functional programming function map in a Javascript context is only an array method as all functional programming javascript method.

For the map data type, see Javascript - Map Data Type


Arrow Function

With an Arrow function as argument

  • Letters that we want to lowercase
const letters = ['A', 'B', 'C', 'D', 'E'];
console.log("Showing only the function value call");
const lowercases = => letter.toLowerCase());
  • With the index
console.log("Showing the value, index call");, index) => console.log("Letter at position "+index+" is "+letter.toLowerCase()));

Function Declaration

With an Javascript - Function Declaration

  • The function declaration
function double(num) {
    return 2*num;
  • The array of number that we want to double.
const numbers = [1, 2, 3, 4, 5];
// Passing the function name
const doubled =;
// Declaring the function inside the map works also
const doubled2 = double(num) {
    return 2*num;
console.log("doubled data");
console.log("doubled2 data");


  • Functional Form, index) => { ... });
  • Procedural form
map((element, index, myArray) => { ... });

map builds a new array that you should use otherwise use other loop construction such as foreach or for…of

Documentation / Reference

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