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++value vs value++

Written byPhuoc Nguyen
15 Jul, 2020
Last updated
22 Aug, 2022
`++` is the increment operator adding one to the operand. There're two variables of the operator which use `++` as prefix or postfix: `++value` and `value++`.
They have the same effect in for loops. The following loops print the same results which are the numbers from 0 to 4:
for (let i = 0; i < 5; i++) {

// And
for (let i = 0; i < 5; ++i) {


When returning a value, `return value++` returns the original value before the value is increased. While `return ++value` increases the value and returns the updated value.
The following `foo` and `bar` functions returns different results:
const foo = (x) => x++;
const bar = (x) => ++x;

foo(1); // 1
bar(1); // 2

Good to know

`value += 1` is another alternative of the postfix increment `value++`. It's worth noting that they can provide different result when using with strings.
The postfix increment `value++` will convert the value to number first, and then increase the value:
let value = '5';
value; // 6
On the other hand, if value is a string, `value += 1` converts the second argument (`1`) to string first, and then concatenate them together.
let value = '5';
value += 1;
value; // '51'

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Phước Nguyễn