Inline `++` in Javascript not working

Question

Astonished to find out that a line like this :

$('#TextBox').val(parseInt($('#TextBox').val())++ );

Will not work !

I've done some tests, it plays out to the conclusion that the inline ++ doesn't work (in Javascript as a whole ? Or simply in my example ?).

Here's a test with three pieces of code, it seems that ++ is ok with a variable but not inline.

So, no inline ++ for us in Javascript?

A cleaner way to do it would be: `$('#TextBox').val(function(){ return parseInt(this.value, 10)+1; });` — Joe, Jul 14 '13 at 16:56
If you want to have some `+` signs together in a confusing way that works, do it like this `1 + + "5"; // 6` — Paul S., Jul 14 '13 at 16:57
take a look at [this](http://stackoverflow.com/questions/3709866/whats-a-valid-left-hand-side-expression-in-javascript-grammar) You're receiving this error in the console: `Reference Error: Invalid left-hand side expression in postfix operation` — kalley, Jul 14 '13 at 16:57
Obviously you failed to research what `++` means. Please tell us, in detail, what you actually expected to happen. — Lightness Races in Orbit, Jul 14 '13 at 17:23
@LightnessRacesinOrbit : I know what it means, I just couldn't see it for some reason, meaning once I got the answer I felt horrible. But I hope this is a good thread for beginners, maybe it'll help some of us. — Akheloes, Jul 14 '13 at 17:27

score 8 · Accepted Answer · answered Jul 14 '13 at 16:54

8

There's nothing particular to jQuery about this. ++ increments a variable. You are trying to increment the return value of a function call.

answered Jul 14 '13 at 16:54

Quentin

Yes. ++ is an operator that only applies to variables and properties of objects. – Andre Pena Jul 14 '13 at 16:56
then why is `parseInt($('#TextBox').val()) + 1` working fine ? – Akheloes Jul 14 '13 at 16:57
2

@Gloserio `++` performs an _assignment_ , where are you changing a property/variable value here? It's for the same reason `1++` doesn't work and `1+1` does. See [the language specification on the Postfix incremenet](http://es5.github.io/#x11.3.1) operator. – Benjamin Gruenbaum Jul 14 '13 at 16:58
@Gloserio — `+` doesn't modify a variable. – Quentin Jul 14 '13 at 16:59
1

I think the "real" reason is that `+ 1` **produces the value of** one more, `++` **post**-increments (returning current value) – Steven Lu Jul 14 '13 at 16:59
God ! I'll go kill myself and let you know ... :( – Akheloes Jul 14 '13 at 16:59

score 4 · Answer 2 · answered Jul 14 '13 at 17:03

4

Q: What does x++ mean?

A: x++ means take the value of x, let's call this n, then set x to be n + 1, then return n.

Q: Why does this fail on a non-variable?

A: Let's try it on something simple, say 3, and see where things go wrong.

Take the value of 3 and call it n, okay, n = 3
Set 3 to be n + 1, so 3 = 3 + 1, 3 = 4 this makes no sense! So if this step can't be done, the ++ operator can't be used.

answered Jul 14 '13 at 17:03

Paul S.

Lols, I can actually even make you even more mad if I tell you I know this :p – Akheloes Jul 14 '13 at 17:04

BrunoLM · Answer 3 · 2013-07-14T17:12:09.870

0

++ works on variables, not directly on numbers

var c = parseInt($('#TextBox').val());
$('#TextBox').val( ++c );

Change the order from

var x = 0;

var result = x++;

result // 0

To

var x = 0;

var result = ++x;

result // 1

Then it will evaluate ++ before retrieving the value.

edited Jul 14 '13 at 17:12

answered Jul 14 '13 at 17:00

BrunoLM

3 Answers3