Published Jun 11 2020
Letβs talk about the similarities first.
null
and undefined
are JavaScript primitive types.
The meaning of undefined
is to say that a variable has declared, but it has no value assigned.
let age //age is undefined
let age = null //age is null
Note: accessing a variable thatβs not been declared will raise a
ReferenceError: <variable> is not defined
error, but this does not mean itβsundefined
.
How do you check if a variable is null? Use the comparison operator, for example age === null
Same for undefined: age === undefined
In both cases, you can check for:
if (!age) {
}
and this will be matching both null
and undefined
.
You can also use the typeof
operator:
let age
typeof age //'undefined'
although null
is evaluated as an object, even though it is a primitive type:
let age = null
typeof age //'object'
I wrote an entire book on this topic π
I also got a super cool course π
© 2023 Flavio Copes
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