BFE.dev solution for JavaScript Quiz
7. Increment Operator

TL;DR

++a and a++ both increments, but returns different values. ++a returns new value while a++ returns the old one.

let a = 1
const b = ++a
// a is 2, b is 2
const c = a++
// a is 3, c is 2
console.log(a)
// 3
console.log(b)
// 2
console.log(c)
// 2

ECMAScript spec for Prefix Increment Operator

From the spec, we can easily understand how it works for ++a.

UpdateExpression : ++ UnaryExpression

1. Let expr be ? Evaluation of UnaryExpression.
2. Let oldValue be ? ToNumeric(? GetValue(expr)).
3. If oldValue is a Number, then
   1. Let newValue be Number::add(oldValue, 1𝔽).
4. Else,
   1. Assert: oldValue is a BigInt.
   2. Let newValue be BigInt::add(oldValue, 1ℤ).
5. Perform ? PutValue(expr, newValue).
6. Return newValue.

Simply the last line, we can see new value is returned.

ECMAScript spec for Postfix Increment Operator

From the spec, we can easily understand how it works for a++.

UpdateExpression : LeftHandSideExpression ++

1. Let lhs be ? Evaluation of LeftHandSideExpression.
2. Let oldValue be ? ToNumeric(? GetValue(lhs)).
3. If oldValue is a Number, then
   1. Let newValue be Number::add(oldValue, 1𝔽).
4. Else,
   1. Assert: oldValue is a BigInt.
   2. Let newValue be BigInt::add(oldValue, 1ℤ).
5. Perform ? PutValue(lhs, newValue).
6. Return oldValue.

We can see that the steps are basically the same as Prefix Increment Operator, except that at last old value is returned.