In the code snippet below, what do a and b evaluate to?

let sum = 0;
const squares = [1, 2, 3, 4, 5].map((x) => (
  (sum += x), x * x
console.log(sum) // 15
console.log(squares) // [1, 4, 9, 16, 25]

This is a fun one. The weirdest part is probably the comma , operator.

If you’re not familiar, , evaluates each of its operands (from left to right) and returns the value of the last operand. This allows us to, in a single line, increase sum by x and return the square of x. When finished, we get the sum of the array as well as a new array of squares.