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up vote 49 down vote favorite
9

In Javascript you can use ++ operator before or after the variable name. What, if any, are the differences between these ways of incrementing a variable?

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up vote 81 down vote accepted

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]
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2  
Curses, I nearly beat you to an answer had I not stopped to load up a practical jsfiddle answer. ;-) – Chris Aug 12 '10 at 16:34
2  
What would this look like if you used + 1 instead of ++? Is there a way to increment before or after when adding numbers? – Keavon Apr 20 '14 at 4:38
up vote 8 down vote
  • ++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.

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up vote 5 down vote

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.

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up vote 1 down vote

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?

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1  
Looking at ECMA-262, it seems reasonably well-specified. – Jon Skeet Aug 12 '10 at 16:51
up vote 0 down vote 
var x = 0, y = 0;

//post-increment: i++ returns value then adds one to it
console.log('x++ will log: ', x++); //0
console.log('x after x++ : ', x);    //1

//pre-increment: adds one to the value, then returns it
console.log('++y will log: ', ++y); //1
console.log('y after ++y : ', y);   //1
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