Question

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Is there a standard function to check for null, undefined, or blank variables in JavaScript?

What is the best way to check for the undefined type in JavaScript? I know one way to check is by using typeOf, but I have to check for it in lots of places; is there a shorter or better way to do this?

I tried a few ways but didn’t succeed:

alert(undefined === "undefined");
alert(undefined || "defined"); 

Answer 1

Nothing new for you:

// either
(val === undefined)
// or
(typeof val == "undefined")

The problem of using val || "defined" is that “defined” will be returned in case val is null, undefined, 0, false or "".