Advertisement
Advertisement


How can I determine if a variable is 'undefined' or 'null'?


Question

How do I determine if variable is undefined or null?

My code is as follows:

var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
  // DO SOMETHING
};
<div id="esd-names">
  <div id="name"></div>
</div>

But if I do this, the JavaScript interpreter halts execution.

2020/03/08
1
2174
3/8/2020 11:15:39 PM

Accepted Answer

You can use the qualities of the abstract equality operator to do this:

if (variable == null){
    // your code here.
}

Because null == undefined is true, the above code will catch both null and undefined.

2017/07/20
2912
7/20/2017 8:11:03 PM


Combining the above answers, it seems the most complete answer would be:

if( typeof variable === 'undefined' || variable === null ){
    // Do stuff
}

This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.

2015/02/09

if (variable == null) {
    // Do stuff, will only match null or undefined, this won't match false
}
2013/12/26

if (typeof EmpName != 'undefined' && EmpName) {

will evaluate to true if value is not:

  • null

  • undefined

  • NaN

  • empty string ("")

  • 0

  • false

2017/12/14

jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.

So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:

if (!EmpName) { //do something }
2010/04/15

I've come to write my own function for this. JavaScript is weird.

It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.

function empty(v) {
    let type = typeof v;
    if (type === 'undefined') {
        return true;
    }
    if (type === 'boolean') {
        return !v;
    }
    if (v === null) {
        return true;
    }
    if (v === undefined) {
        return true;
    }
    if (v instanceof Array) {
        if (v.length < 1) {
            return true;
        }
    } else if (type === 'string') {
        if (v.length < 1) {
            return true;
        }
        if (v === '0') {
            return true;
        }
    } else if (type === 'object') {
        if (Object.keys(v).length < 1) {
            return true;
        }
    } else if (type === 'number') {
        if (v === 0) {
            return true;
        }
    }
    return false;
}

TypeScript-compatible.


This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)

Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.

"0.0", NaN, " ", true are considered non-empty.

2020/03/08

Source: https://stackoverflow.com/questions/2647867
Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow
Email: [email protected]