The Difference Between i++ and ++i (Postfix vs. Prefix)

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JavaScript (and many other languages) support the postfix and the prefix increment operator (++). You have probably seen and used it before.

Often it's used like this:

i++;

In this case it's almost equivalent to:

i = i + 1;

But, what do you think? Is there a difference between

let i = 3;
const j = i++;

and

let i = 3;
const j = ++i;

...

Well, yes. The first example uses the postfix increment operator (i++). The second example uses the prefix increment operator (++i). At first, it seems like there's no difference. However, it's important to understand what is going on here:

The postfix increment operator increments the value and returns the value before the increment.

The prefix increment operator increments the value and returns the value after the increment.

Let's take a look at our two examples again:

// postfix increment

let i = 3;
const j = i++;

console.log({ i, j }); // { i: 4, j: 3 }
// prefix increment

let i = 3;
const j = ++i;

console.log({ i, j }); // { i: 4, j: 4 }

Spotted the difference? The value of j differs. Therefore, it is important to know this small difference between postfix and prefix.

By the way, the same applies to the postfix decrement and prefix decrement operator (--). The only difference is, that instead of incrementing we are decrementing the value.

That's all there is to say. I hope I made the difference a bit clearer. See you soon!


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