Coding - JS utils
Array Prototype Map

Difficulty: Easy, Duration: 15 minutes, Languages: JS, TS, Author: Kiran Dash (opens in a new tab)

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Question is a powerful utility function in JavaScript that allows you to transform each element of an array using a callback function. In this exercise, you will implement a simplified version of


const double = (x: number) => x * 2;
const input = [1, 2, 3];
input.myMap(double); // [2, 4, 6]

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with .call() method

Array.prototype.myMap = function (callbackFn, thisArg) {
  if (typeof callbackFn !== "function" || ! {
    throw new TypeError(`${callbackFn} is not a function`);
  // input array can be accessed using this keyword
  const len = this.length;
  const outputArr = Array(len);
  for (let k = 0; k < len; k++) {
    // Object.hasOwn is used to handle the edge case for sparse array and avoid getting NaN in result
    if (Object.hasOwn(this, k)) {
      outputArr[k] =, this[k], k, this);
  return outputArr;

Edge Cases:

  • Handling Sparse arrays ex: [1, , 3] where this.length will be 3 but this[1] will be undefined. And we should not call callbackFn for undefined values. We handled this by using Object.hasOwn(this, k) in the for loop.
  • Handling index and array as fn params: Calling the callbackFn using .call method and pass index and array as arguments.
  • Handling thisArg: If thisArg is passed, the callbackFn should not be an arrow function, as arrow functions do not have their own this context.
  • Handling type check: If callbackFn is not a function or does not have a call method, we throw a TypeError.

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