var w = q||q2; what does this expression means?

Question

what does this means?

var q = [];
var q2 = ["canada","usa","mexico"];
var w = q||q2;
document.writeln(w);

the value of the variable w is : [] the empty list.

can someone explain to me why it is displaying this [] instead of ["canada","usa","mexico"].

Because `[]` is an array object which is considered a truthy value. `q.length ? q : q2;` — Ram, Apr 05 '15 at 23:22
Not empty; falsy. But `q` is truthy, so `q||q2` will evaluate to `q` — elclanrs, Apr 05 '15 at 23:22

aug · Answer 1 · 2015-04-06T00:17:43.253

You should read Logical Operators from MDN. According to the documentation

The logical operator OR (||) expr1 || expr2

Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.

In a nutshell, logical operators evaluate from left to right. In your situation, since you have declare q as an empty array ([]), it evaluates to true and immediately goes and assigns w to that.

If you want q2 to take precedence, you can simply do

var w = q2 || q;

so that way only if q2 evaluates to falsey will it assign w to be q instead. The other option is to not declare q at all or declare it as something falsey. You can find out what evaluates to false here.

var w = q||q2; what does this expression means?

1 Answers1