JavaScript || or operator with an undefined variable

Question

I have been doing some reading lately one article I read was from Opera.

http://dev.opera.com/articles/view/javascript-best-practices/

In that article they write this:

Another common situation in JavaScript is providing a preset value for a variable if it is not defined, like so:

if(v){   var x = v; } else {   var x = 10; } 

The shortcut notation for this is the double pipe character:

var x = v || 10; 

For some reason, I can't get this to work for me. Is it really possible to check to see if v is defined, if not x = 10?

--Thanks. Bryan

Answer 1

That Opera article gives a poor description of what is happening.

While it is true that x will get the value of 10 if v is undefined. It is also true that x will be 10 if v has any "falsey" value.

The "falsey" values in javascript are:

• 0
• null
• undefined
• NaN
• "" (empty string)
• false

So you can see that there are many cases in which x will be set to 10 besides just undefined.

Here's some documentation detailing the logical operators. (This one is the "logical OR".) It gives several examples of its usage for such an assignment.

Quick example: http://jsfiddle.net/V76W6/

var v = 0;  var x = v || 10;  alert( x ); // alerts 10 

Assign v any of the falsey values that I indicated above, and you'll get the same result.