Both null
and undefined are used to represent a variable with no value. But there is a
small difference between these two.
undefined
If you
declare a variable without assigning any value, then it is treated as
undefined.
null
You can
assign null to any variable to specify that variable has no value, but you
can’t assign undefined to any variable.
One more
difference is type of null return object, where as type of undefined return
undefined.
Comparison of null and undefined
null === undefined // Strict comparison
operator return false
null == undefined // true
null == null // true
undefined
== undefined //true
undefined.html
<!DOCTYPE html> <html> <head> <title>null Vs undefined</title> </head> <body> <script> var a; var b = null; document.write("a = " + a + " Type of a = " + (typeof a) + "<br />"); document.write("b = " + b + " Type of b = " + (typeof b) + "<br />"); document.write("a == b : " + (a == b) + "<br />"); document.write("a === b : " + (a === b) + "<br />"); document.write("a == a : " + (a == a) + "<br />"); document.write("b == b : " + (b == b) + "<br />"); </script> </body> </html>
Open above
page in browser, you can able to see following text.
a =
undefined Type of a = undefined
b = null
Type of b = object
a == b :
true
a === b :
false
a == a :
true
b == b :
true
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