JS Map

Brushing up on JavaScript’s .map() method today. I realized I was asking prospective hires at work a question about .map() that I didn’t know the answer to myself—not off the top of my head, at least.

The .map() method on an array in JavaScript creates a new array, filled with the results of running a provided callback function on each array element. So [1,2,3].map(x => x*2) returns [2, 4, 6].

.map() can take two parameters: the callback function and a value to use as this when executing the callback function.

The callback function has three optional parameters: element, index and array. The element is the array element you manipulate with each iteration. The index is the (zero-based) count of how many iterations have happened so far. The array is the original array.

.map() callback parameter

element parameter

console.log([1,2,3].map(x => x*2)) outputs: [2, 4, 6] // [1*2, 2*2, 3*2]

index parameter

console.log([1,2,3].map((x,i) => x*i)) outputs: [0, 2, 6] // [1*0, 2*1, 3*2]

array parameter

console.log([1,2,3].map((x,i,arr) => {
  console.log(arr)
  return x*i
}))

outputs:

[ 1, 2, 3 ]
[ 1, 2, 3 ]
[ 1, 2, 3 ]
[ 0, 2, 6 ]

.map() thisArg parameter

ChatGPT gave me this nice example of a simple use of the thisArg parameter. In this exapmle, multiplier is the thisArg value:

const multiplier = {
  factor: 2,
  multiply(num) {
    return num * this.factor;
  }
};

const numbers = [1, 2, 3];

const doubled = numbers.map(function(num) {
  return this.multiply(num);
}, multiplier); // Passing multiplier as thisArg

console.log(doubled); // outputs [2, 4, 6]

Sources

• MDN, “Array.prototype.map()”. accessed 2/20/25
• I used ChatGPT 4o for the thisArg example. accessed 2/20/25