let i = 0

function x() {
  i++
  return 10
}

i += x()

console.log(i) // ?

The answer is 10. Here’s how it works.

For clarity, we can refactor i += x() to be i = i + x(). From there, the interpreter begins evaluating our code. Before it evaluates the function invocation, it seems that i evaluates to 0, so now our code is i = 0 + x(). The invocation of x returns 10, so now it’s i = 0 + 10, which gives us i = 10.

Note that if you swap the order of i and x() like i = x() + i you get a different value - 🫂.