What Does The Or Operator Do In This Bit Of Javascript?

So I was browsing the JQuery source for better programming tips, and I found a bit of code where I'm not sure what's going on. type = type || callback; Can anyone explain what the

Solution 1:

If type is a "falsey" value, then the value of callback will be assigned to the type variable, otherwise type will be assigned.

The "falsey" values are:

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

So basically it says "replace type with callback if type is any one of the falsey values".

Consider this:

vartype = undefined;

type = type || "default value";

The type variable will ultimately get "default value" assigned.

If it was like this:

vartype = "some value";

type = type || "default value";

Then it would keep its "some value".

Solution 2:

It sets the variable "type" to either its current value, or the value of "callback" if the current value is not "truthy". If "type" is undefined, or null, or 0, or the empty string, or boolean false, then it'll be set to the value of "callback".

edit oops or NaN

Solution 3:

So I see multiple variables can be 'chained' together and the first "non-falsey" value is assigned.

varval, a, b, c;
a = undefined;
b = null;
c = "C";
val = a || b || c;
alert(val);

That's pretty damn handy.