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);
            


        

Answer 1

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
}

Answer 2

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; }

Answer 3

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.