In JavaScript, you can check whether a variable is undefined or null or blank or has a value defined in it.
In JavaScript, ‘undefined’ means a variable has been declared but has not been assigned a value yet, such as:
var a; console.log(a); //shows undefined console.log(typeof a); //shows undefined
‘null’ is an assignment value. This can be assigned to a variable as a representation of no value:
var a = null; console.log(a); //shows null console.log(typeof a); //shows object
So, from the preceding examples, this is clear that undefined and null are two distinct types i.e. undefined is a type itself where as null is an object.
null === undefined // false null == undefined // true null === null // true
and,
null = 'value' // ReferenceError undefined = 'value' // 'value'
So, null and undefined are both are used to represent the absence of some value.
You can just check whether the variable has a true value or not.
That means:
if( value ) {
...
}Read Also: Getting Started with Handlebars JS
This will evaluate to true if value is not:
– null
– undefined
– NaN
– empty string (“”)
– 0
– false
If you do not know whether a variable exists or not i.e. if it was declared you should check with the typeof operator.
For instance,
if( typeof value !== 'undefined' ) {
...
}If you know that a variable is declared at least, then, you should directly check with the value also.
It seems the most complete answer would be:
if( typeof value === 'undefined' || variable === null ){
// Do stuff
}Read Also: Date Object – JavaScript
In modern browsers you can safely compare the variable directly to undefined:
if (value === undefined) {...}You can use void operator to get the value of undefined.
This will work even if the global window.undefined value has been over-written:
if (value === void(0)) {...}But don’t use void(0) directly.
Use instead:
var undefined = void(0); console.log(value === undefined);
Although null can do things undefined does, this is more or less related to objects rather than scalars.
Indeed, JavaScript considers null itself an object — typeof null returns “object”.
Based on United States, Jacob Frazier is a skilled JavaScript developer with over 8 years of experience. He is passionate about change and trying new things, both professionally and personally. He loves startups and is extremely proactive.