// bad (albeit way faster)

const arr = [1, 2, 3, 4];

const len = arr.length;

var i = -1;

var result = [];

while (++i < len) {

var n = arr[i];

if (n % 2 > 0) continue;

result.push(n * n);

}

// good

const arr = [1, 2, 3, 4];

const isEven = n => n % 2 == 0;

const square = n => n * n;

const result = arr.filter(isEven).map(square);