How to detect if a JS variable is undefined?

Question

// mystring contains dynamic text. Sometimes can be
// null
var a = mystring.split(" ");
var proc = a[0] + " " + a[1];

If a does not contain text, after split proc is undefined. If I try to assing its value to a textbox the result is "undefined":

mytextbox.val(proc);

enter image description here

So I need a way to make that proc has always a value, at least an empty string.

@a_maar: if( mystring && mystring.length ) { // all the rest } — jAndy
– jAndy, Commented Nov 29, 2012 at 17:08
@Jack: that would fail because you cannot call split on mystring when mystring is undefined — hereandnow78
– hereandnow78, Commented Nov 29, 2012 at 17:10
Is mystring really null? I think you mean the empty string. — Bergi
– Bergi, Commented Nov 29, 2012 at 17:10

Xymostech · Accepted Answer · 2012-11-29 17:06:02Z

2

You can just use

(mystring || " ")

which will evaluate to mystring if it is not null, or " " if it is.

Or, you can just put an if statement around the whole thing:

if (mystring != null) {
    // stuff
} else {
    var proc = "";
}

answered Nov 29, 2012 at 17:06

Xymostech

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1 Comment

jAndy Over a year ago

not a good usecase for a logical OR here imho. It shouldn't do all the logic if mystring is undef or empty.

Bergi · Accepted Answer · 2012-11-29 17:25:12Z

2

var proc = "";
if (mystring !== null) { // omit this if you're sure it's a string
    var a = mystring.split(" ");
    if (a.length > 1) {
        proc = a[0] + " " + a[1];
    }
}

I'm quite sure your proc is not undefined, but the string " undefined".

edited Nov 29, 2012 at 17:25

answered Nov 29, 2012 at 17:09

Bergi

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2 Comments

jAndy Over a year ago

why negating the logic ? Reads positive much clearer, no ? if( mystring && mystring.length)

Bergi Over a year ago

@jAndy: You mean the null logic? I'm still not sure whether the OP really has null or just an empty string, so I wanted to make it explicit. Btw, mystring.length is not enought to check to prevent a[1] from being undefined.

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