Null and undefined: What's the difference?
November 16, 2020
The two types null and undefined seem similar. They're both a kind of nothing — falsy values they aren't a number or string.
But what is the difference between these two types of values?
The official spec of JavaScript, called the Ecma Standard, says this about null:
"The Null type has exactly one value, called null."
OK, so the Null type can only have a value of null. But what does that value represent? MDN says it represents "the intentional absence of any object value."
APIs can return null where an object is expected, but no object meets the criteria provided to the API.
As for undefined, the spec says this:
"The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined."
So when a variable is declared but not yet assigned a value, it automatically has a value of undefined.
Also, when a function does not return a value, it returns undefined.
// a variable is declared, but not assigned a value
var foo
console.log(foo)
// undefined
foo === undefined
// true
!!foo
// false
// a function does not return a value
function bar() {
1 + 2
}
console.log(bar())
// undefined
Undefined and null are among JavaScript's seven primitive types. (Remember, a primitive type is a value that is not an object and has no methods. Also, primitive values are immutable.) Those seven types are:
- String
- Number
- Bigint
- Boolean
- Symbol
- Undefined
- Null
But wait! MDN actually says there are six primitives, which excludes null. It calls null a "special case." The typeof operator shows that null is, in fact, an object:
typeof null
// "object"
This post by Dr. Axel Rauschmayer shows that this is actually a bug from the first version of JavaScript that has never been fixed.
In that version, types were denoted by a type tag as well as the actual data of the value. null was represented by the NULL pointer, which is 0x00. In that case, 0 is the type tag, which is the type tag of an object.
However, MDN also notes that every Object is derived from the null value, so it makes sense that null is defined as an object.
"Every Object is derived from null value, and therefore typeof operator returns object for it."
To sum it up, null is the intentional absence of value. undefined is a value for something that does not exist yet or does not exist anymore.