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

The best answers to the question “Is there a standard function to check for null, undefined, or blank variables in JavaScript?” in the category Dev.

QUESTION:

Is there a universal JavaScript function that checks that a variable has a value and ensures that it’s not undefined or null? I’ve got this code, but I’m not sure if it covers all cases:

function isEmpty(val){
    return (val === undefined || val == null || val.length <= 0) ? true : false;
}

ANSWER:

The verbose method to check if value is undefined or null is:

return value === undefined || value === null;

You can also use the == operator but this expects one to know all the rules:

return value == null; // also returns true if value is undefined

ANSWER:

You can just check if the variable has a truthy value or not. That means

if( value ) {
}

will evaluate to true if value is not:

  • null
  • undefined
  • NaN
  • empty string (“”)
  • 0
  • false

The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.

Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance

if( typeof foo !== 'undefined' ) {
    // foo could get resolved and it's defined
}

If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

ANSWER:

This is the safest check and I haven’t seen it posted here exactly like that:

if (typeof value !== 'undefined' && value) {
    //deal with value'
};

It will cover cases where value was never defined, and also any of these:

  • null
  • undefined (value of undefined is not the same as a parameter that was never defined)
  • 0
  • “” (empty string)
  • false
  • NaN

Edited: Changed to strict equality (!==) because it’s the norm by now 😉

ANSWER:

function isEmpty(value){
  return (value == null || value.length === 0);
}

This will return true for

undefined  // Because undefined == null

null

[]

""

and zero argument functions since a function’s length is the number of declared parameters it takes.

To disallow the latter category, you might want to just check for blank strings

function isEmpty(value){
  return (value == null || value === '');
}