What a strange syntax?

Question

I've found unknown for me code construction on JQuery site. After some formatting it looks like:

function (a,c) {
    c==null && (c=a,a=null);
    return arguments.length>0
        ? this.bind(b,a,c) 
        : this.trigger(b)
}

What does the first line of the function mean? Is it any trick or standard JS code construction?

it's a standard in Java, php, Javascript and about any other language — Tom, Jul 26 '11 at 12:07
If you're going to look at the jQuery code, look at the development copy and not the minified one. **[Here's a link](http://code.jquery.com/jquery-1.6.2.js)** to the development copy. — qwertymk, Jul 26 '11 at 12:28
@Tom: the [comma operator](http://ecma262-5.com/ELS5_HTML.htm#Section_11.14) is different in JavaScript that in other languages with a C-inspired syntax. — dolmen, Jul 26 '11 at 12:38
Wow, those code minifiers are aggressive - it's only one character shorter than `if(c==null){c=a;a=null}` — Random832, Jul 26 '11 at 13:33

score 15 · Accepted Answer · answered Jul 26 '11 at 12:06

15

It's a trick that uses boolean short-circuit evaluation to only do the second half if the first evaluates to true. Perl has this commonly:

<something> or die

where if the first statement failed, the program ends.

Read it as

if (c == null) { c = a; a = null; }

answered Jul 26 '11 at 12:06

dascandy

2

The comma makes both commands execute without need of a semicolon. – Digital Plane Jul 26 '11 at 12:08
2

which in turn allows *both* statements to execute in a **single line**, which is a character savings over an if-statement. – zzzzBov Jul 26 '11 at 13:19
2

@Gunslinger_ : how much faster actually? – Mchl Jul 26 '11 at 15:07
Python also allows you to use `A or B` as an equivalent to something like `A != null && A == true ? A : B`. Though in Python you'd say that as `A if A != None and A == True else B`. I might be missing some of the subtleties of Python's usage, though. – JAB Jul 26 '11 at 16:23
@JAB that is possible in javascript – Jul 26 '11 at 21:35

score 12 · Answer 2 · answered Jul 26 '11 at 12:06

12

That's an ugly way to write

if(c==null) {
  c = a;
  a = null;
}

This utilizes the fact, that the second part of boolean && will be executed if, and only if the first part evaluates to true.

answered Jul 26 '11 at 12:06

Mchl

2

Very ugly. And very unintuitive to many developers. Can you find the person who wrote it and kick them. – Schroedingers Cat Jul 26 '11 at 12:09
10

@Schroedingers Cat: as variables are named `a`, `b`, `c` this code has probably been generated by a code minifier. [jQuery currently seems](https://github.com/jquery/jquery/blob/master/Makefile#L8) to use [UglifyJS](https://github.com/mishoo/UglifyJS). – dolmen Jul 26 '11 at 12:48
2

Uglify? That explains a lot, doesn't it? ;D – Mchl Jul 26 '11 at 15:06

score 2 · Answer 3 · answered Jul 26 '11 at 12:39

The expression uses two JavaScript features :

short circuit evaluation of boolean operators: in statement context, a && (b); is equivalent to if (a) (b);
the comma operator to group assignment expressions: in statement context, a=b,b=c; is equivalent to { a=b; b=c }

As a result the expression is equivalent to:

if (c == null) {
    c = a
    a = null
}

3 Answers3