How do I check a variable if it's null
or undefined
and what is the difference between the null
and undefined
?
What is the difference between ==
and ===
(it's hard to search Google for "===" )?
This question is related to
javascript
null
undefined
The spec is the place to go for full answers to these questions. Here's a summary:
x
, you can:null
by direct comparison using ===
. Example: x === null
undefined
by either of two basic methods: direct comparison with undefined
or typeof
. For various reasons, I prefer typeof x === "undefined"
.null
and undefined
by using ==
and relying on the slightly arcane type coercion rules that mean x == null
does exactly what you want.==
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:
Ad 1. null
is not an identifier for a property of the global object, like undefined
can be
let x; // undefined_x000D_
let y=null; // null_x000D_
let z=3; // has value_x000D_
// 'w' // is undeclared_x000D_
_x000D_
if(!x) console.log('x is null or undefined');_x000D_
if(!y) console.log('y is null or undefined');_x000D_
if(!z) console.log('z is null or undefined');_x000D_
_x000D_
try { if(w) 0 } catch(e) { console.log('w is undeclared') }_x000D_
// typeof not throw exception for undelared variabels_x000D_
if(typeof w === 'undefined') console.log('w is undefined');
_x000D_
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 !==)
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) then
optional 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
How do I check a variable if it's null or undefined
just check if a variable has a valid value like this :
if(variable)
it will return true if variable does't contain :
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 .
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
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);
}
}
Source: Stackoverflow.com