What does mean && between variables in line without if?

Question

I have following line of code in javascript:

q && (c = q === "0" ? "" : q.trim());

WHat does it mean? I understand that c is equals either empty string or q.trim() result, but what does mean q && ()?

score 5 · Accepted Answer · answered Sep 13 '12 at 13:54

5

JavaScript optimizes boolean expressions. When q is false, the right hand side of the expression doesn't matter for the result, so it's not executed at all. So this is a short form of:

if( q ) {
    c = q === "0" ? "" : q.trim()
}

answered Sep 13 '12 at 13:54

Aaron Digulla

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4

I think you mean "if (q)" ... – tucuxi Sep 13 '12 at 13:55

score 3 · Answer 2 · answered Sep 13 '12 at 13:54

It is a guard against undefined/null/false variables.

If q is not defined, false or set to null, the code short circuits and you don't get errors complaining about null.trim()

Another way to write that would be:

if(q) {
  c = q === "0" ? "" : q.trim();
}

score 0 · Answer 3 · answered Sep 13 '12 at 13:54

0

You can use this kind of clause to check wether q is defined.

answered Sep 13 '12 at 13:54

kisp

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What does mean && between variables in line without if?

3 Answers3