undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefinednull is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows objectFrom the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
undefined means a variable has been declared but has not yet been assigned a value :
var testVar;
console.log(testVar); //shows undefined
console.log(typeof testVar); //shows undefinednull is an assignment value. It can be assigned to a variable as a representation of no value :
var testVar = null;
console.log(testVar); //shows null
console.log(typeof testVar); //shows objectFrom the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
Proof :
console.log(null === undefined) // false (not the same type)
console.log(null == undefined) // true (but the "same value")
console.log(null === null) // true (both type and value are the same)and
null = 'value' // Uncaught SyntaxError: invalid assignment left-hand side
undefined = 'value' // 'value'
The difference can be explained with toilet tissue holder:
A non-zero value is like a holder with roll of toilet tissue and there's tissue still on the tube.
A zero value is like a holder with an empty toilet tissue tube.
A null value is like a holder that doesn't even have a tissue tube.
An undefined value is similar to the holder itself being missing.
Undefined vs null
null vs undefined
JavaScript undefined vs. null
:)
ya, i know.. glad someone passed on the wisdom.
the way i always keep it in mind is to just consider the fact that both null and undefined are more falsish than they are trueish..
i know, all the hardcore C heads are going to scream "NULL IS UNKNOWN ITS NOT TRUE OR FALSE!"
but that doesn't matter.
what you are thinking as a normal human, and not a machine. Null is closer to the idea of a non-thing than it is to a thing. Therefor whenever you just want to know if something exists or has value you can say:
if (thing) alert(thing);
but, be careful when your var is a number, because 0 will fail this test too. the simplest way to keep all this strait is to just keep your numeric primitives sep from your objects.
More on reddit.comNot sure when to use Undefined and Null in JavaScript?
When the typeof keyword is used on undefined it outputs "undefined" which is correct. When typeof is used on null it outputs "object "which is not correct and is a flaw in the language that was never fixed. Therefore it is a perfectly acceptable practice to simply never use null and always use undefined. You can use null if you want to but there is no reason to. Here is a reference: https://www.youtube.com/watch?v=vJKDh4UEXhw&feature=youtu.be&t=38m60s
More on reddit.comVideos
Since switching to TypeScript I have been using a lot of optional properties, for example:
type store = {
currentUserId?: string
}
function logout () {
store.currentUserId = undefined
}However my coworkers and I have been discussing whether null is a more appropriate type instead of undefined, like this:
type store = {
currentUserId: string | null
}
function logout () {
store.currentUserId = null
}It seems like the use of undefined in TypeScript differs slightly from in Javascript.
Do you guys/girls use undefined or null more often? And, which of the examples above do you think is better?