Javascript comma operator

Question

When combining assignment with comma (something that you shouldn't do, probably), how does javascript determine which value is assigned? Consider these two snippets:

function nl(x) { document.write(x + "<br>"); }
var i = 0;
nl(i+=1, i+=1, i+=1, i+=1);
nl(i);

And:

function nl(x) { document.write(x + "<br>"); }
var i = 0;
nl((i+=1, i+=1, i+=1, i+=1));
nl(i);

The first outputs

1
4

while the second outputs

4
4

What are the parentheses doing here?

EricLaw · Accepted Answer · 2010-04-09 23:57:21Z

3

I was confusing two things, here. The first call to 'nl' is a function call with four arguments. The second is the evaluation of the comma into one argument.

So, the answer: the value of a list of expressions separated by ',' is the value of the last expression.

edited Apr 9, 2010 at 23:57

EricLaw

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answered Feb 2, 2009 at 3:53

Claudiu

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1 Comment

Shog9 Over a year ago

Well, at least you have a nice little "what does this code do" question now. :)

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