JavaScript checking for null vs. undefined and difference between == and ===

Question

1. How do I check a variable if it\'s null or undefined and what is the difference between the null and undefined?<

Answer 1

Try With Different Logic. You can use bellow code for check all four(4) condition for validation like not null, not blank, not undefined and not zero only use this code (!(!(variable))) in javascript and jquery.

function myFunction() {
var data;  //The Values can be like as null, blank, undefined, zero you can test

if(!(!(data)))
{
   //If data has valid value
    alert("data "+data);
} 
else 
{
    //If data has null, blank, undefined, zero etc.
    alert("data is "+data);
}

}

Answer 2

Ad 1. null is not an identifier for a property of the global object, like undefined can be

let x;      // undefined
let y=null; // null
let z=3;    // has value
// 'w'      // is undeclared

if(!x) console.log('x is null or undefined');
if(!y) console.log('y is null or undefined');
if(!z) console.log('z is null or undefined');

try { if(w) 0 } catch(e) { console.log('w is undeclared') }
// typeof not throw exception for undelared variabels
if(typeof w === 'undefined') console.log('w is undefined');

Ad 2. The === check values and types. The == dont require same types and made implicit conversion before comparison (using .valueOf() and .toString()). Here you have all (src):

if

== (its negation !=)

=== (its negation !==)

Answer 3

undefined

It means the variable is not yet intialized .

Example :

var x;
if(x){ //you can check like this
   //code.
}

equals(==)

It only check value is equals not datatype .

Example :

var x = true;
var y = new Boolean(true);
x == y ; //returns true

Because it checks only value .

Strict Equals(===)

Checks the value and datatype should be same .

Example :

var x = true;
var y = new Boolean(true);
x===y; //returns false.

Because it checks the datatype x is a primitive type and y is a boolean object .

Answer 4

The difference is subtle.

In JavaScript an undefined variable is a variable that as never been declared, or never assigned a value. Let's say you declare var a; for instance, then a will be undefined, because it was never assigned any value.

But if you then assign a = null; then a will now be null. In JavaScript null is an object (try typeof null in a JavaScript console if you don't believe me), which means that null is a value (in fact even undefined is a value).

Example:

var a;
typeof a;     # => "undefined"

a = null;
typeof null;  # => "object"

This can prove useful in function arguments. You may want to have a default value, but consider null to be acceptable. In which case you may do:

function doSomething(first, second, optional) {
    if (typeof optional === "undefined") {
        optional = "three";
    }
    // do something
}

If you omit the optional parameter doSomething(1, 2) thenoptional will be the "three" string but if you pass doSomething(1, 2, null) then optional will be null.

As for the equal == and strictly equal === comparators, the first one is weakly type, while strictly equal also checks for the type of values. That means that 0 == "0" will return true; while 0 === "0" will return false, because a number is not a string.

You may use those operators to check between undefined an null. For example:

null === null            # => true
undefined === undefined  # => true
undefined === null       # => false
undefined == null        # => true

The last case is interesting, because it allows you to check if a variable is either undefined or null and nothing else:

function test(val) {
    return val == null;
}
test(null);       # => true
test(undefined);  # => true

Answer 5

The spec is the place to go for full answers to these questions. Here's a summary:

1. For a variable x, you can:
   
   
   • check whether it's null by direct comparison using ===. Example: x === null
   • check whether it's undefined by either of two basic methods: direct comparison with undefined or typeof. For various reasons, I prefer typeof x === "undefined".
   • check whether it's one of null and undefined by using == and relying on the slightly arcane type coercion rules that mean x == null does exactly what you want.
     
     
2. The basic difference between == and === is that if the operands are of different types, === will always return false while == will convert one or both operands into the same type using rules that lead to some slightly unintuitive behaviour. If the operands are of the same type (e.g. both are strings, such as in the typeof comparison above), == and === will behave exactly the same.

More reading:

• Angus Croll's Truth, Equality and JavaScript
• Andrea Giammarchi's JavaScript Coercion Demystified
• comp.lang.javascript FAQs: JavaScript Type-Conversion

Answer 6

If your (logical) check is for a negation (!) and you want to capture both JS null and undefined (as different Browsers will give you different results) you would use the less restrictive comparison: e.g.:

var ItemID = Item.get_id();
if (ItemID != null)
{
 //do stuff
}

This will capture both null and undefined