++someVariable vs. someVariable++ in JavaScript

Question

In JavaScript you can use ++ operator before (pre-increment) or after the variable name (post-increment). What, if any, are the differences between these ways of incrementing a variable?

Solution 1

Same as in other languages:

• ++x (pre-increment) means "increment the variable; the value of the expression is the final value"
• x++ (post-increment) means "remember the original value, then increment the variable; the value of the expression is the original value"

Now when used as a standalone statement, they mean the same thing:

x++;
++x;

The difference comes when you use the value of the expression elsewhere. For example:

x = 0;
y = array[x++]; // This will get array[0]

x = 0;
y = array[++x]; // This will get array[1]

Solution 2

• ++x increments the value, then evaluates and stores it.
• x++ evaluates the value, then increments and stores it.

var n = 0, m = 0;

alert(n++); /* Shows 0, then stores n = 1 */
alert(++m); /* Shows 1, then stores m = 1 */

Note that there are slight performance benefits to using ++x where possible, because you read the variable, modify it, then evaluate and store it. Versus the x++ operator where you read the value, evaluate it, modify it, then store it.

Solution 3

As I understand them if you use them standalone they do the same thing. If you try to output the result of them as an expression then they may differ. Try alert(i++) as compared to alert(++i) to see the difference. i++ evaluates to i before the addition and ++i does the addition before evaluating.

See http://jsfiddle.net/xaDC4/ for an example.

Solution 4

I've an explanation of understanding post-increment and pre-increment. So I'm putting it here.

Lets assign 0 to x

let x = 0;

Lets start with post-increment

console.log(x++); // Outputs 0

Why?

Lets break the x++ expression down

x = x;
x = x + 1;

First statement returns the value of x which is 0

And later when you use x variable anywhere, then the second statement is executed

Second statement returns the value of this x + 1 expression which is (0 + 1) = 1

Keep in mind the value of x at this state which is 1

Now lets start with pre-increment

console.log(++x); // Outputs 2

Why?

Lets break the ++x expression down

x = x + 1;
x = x;

First statement returns the value of this x + 1 expression which is (1 + 1) = 2

Second statement returns the value of x which is 2 so x = 2 thus it returns 2

Hope this would help you understand what post-increment and pre-increment are!

Solution 5

var a = 1;
var b = ++a;
alert('a:' + a + ';b:' + b); //a:2;b:2

var c = 1;
var d = c++;
alert('c:' + c + ';d:' + d); //c:2;d:1

jsfiddle

View more solutions

Comments

[Bergi]

See also the language-agnostic Difference between i++ and ++i in a loop?

[palswim]

I was thinking about this yesterday reading this response to the question about bad assumptions in C/C++. In all cases, can we guarantee that Javascript behaves this way? Or do you think it's bad practice to use the increment statement within a more complex statement at all?

[palswim]

The previous comment is actually a copy of an answer (a non-answer, rather) I posted in 2010. I have deleted the answer, but Jon Skeet had replied with: "Looking at ECMA-262, it seems reasonably well-specified."

[shieldgenerator7]

@palswim It's better to use them as standalone statements, and not within a more complex statement. Keeping it by itself improves readability and reduces confusion

[Chris]

Curses, I nearly beat you to an answer had I not stopped to load up a practical jsfiddle answer. ;-)

[Keavon]

What would this look like if you used + 1 instead of ++? Is there a way to increment before or after when adding numbers?

[Jean Jimenez]

I would like to know why if you do this operation const r1 =(x++)+(x++); it does not produce the expected result according to your example.

[Jon Skeet]

@JeanJimenez: Well it produces the result I expect. For example, if x starts off as 10, the value of r1 is 21, which is 10+11. The value of the first x++ expression is 10 and x is incremented to 11. The value of the second x++ expression is 11 and x is incremented to 12.

[Jean Jimenez]

Dear @JonSkeet thanks for that super-fast response, I’m new to learning JavaScript and my confusion is regarding why one increments and the another doesn't.

[Jon Skeet]

@JeanJimenez: They both increment x (which is why x ends up as 12 afterwards if it starts off as 10), and in both cases the value of the expression is the value of x before that increment. (But the second x++ is evaluated after the first increment takes place, of course...)

[danjah]

I um, have been working with JS for many years, and I uh... never knew ++variable was a thing. I cannot think of a single time I've seen it used until I read an article on functional programming and saw it in a loop comparison example. So great question :D

[gdibble]

💭 Additional info for i++ vs. ++i in loops @ stackoverflow.com/questions/29885719/…

[shieldgenerator7]

I highly recommend to only use this syntax as a standalone statement. If you use it inside other expressions it just causes confusion and makes it hard to read.