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
Undefined vs null
javascript - Falsy values vs null, undefined, or empty string - Software Engineering Stack Exchange
Is my interpretation of null vs. undefined correct?
This does not completely cover all cases.
Undefined doesn't necessarily mean that a variable exists but hasn't been given a value yet. It's also possible to explicitly give a variable the value of "undefined", and it is sometimes returned when trying to look up an object member that doesn't exist. E.g. when looking up myObj.thisDoesntExist.
Variables that are set to null do return a value: the value being null.
They need to both be taken into account because they behave slightly differently. For example, if you have a function with default arguments, the default argument will be used if you pass undefined (or no argument at all) - but if you pass null, then your null will be used.
Videos
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?
In programming, truthiness or falsiness is that quality of those boolean expressions which don't resolve to an actual boolean value, but which nevertheless get interpreted as a boolean result.
In the case of C, any expression that evaluates to zero is interpreted to be false. In Javascript, the expression value in
if(value) {
}
will evaluate to true if value is not:
null
undefined
NaN
empty string ("")
0
false
See Also
Is there a standard function to check for null, undefined, or blank variables in JavaScript?
The set of "truthy" and "falsey" values in JavaScript comes from the ToBoolean abstract operation defined in the ECMAScript spec, which is used when coercing a value to a boolean:
+--------------------------------------------------------------------------+
| Argument Type | Result |
|---------------+----------------------------------------------------------|
| Undefined | false |
|---------------+----------------------------------------------------------|
| Null | false |
|---------------+----------------------------------------------------------|
| Boolean | The result equals the input argument (no conversion). |
|---------------+----------------------------------------------------------|
| Number | The result is false if the argument is +0, −0, or NaN; |
| | otherwise the result is true. |
|---------------+----------------------------------------------------------|
| String | The result is false if the argument is the empty String |
| | (its length is zero); otherwise the result is true. |
|---------------+----------------------------------------------------------|
| Object | true |
+--------------------------------------------------------------------------+
From this table, we can see that null and undefined are both coerced to false in a boolean context. However, your fields.length === 0 does not map generally onto a false value. If fields.length is a string, then it will be treated as false (because a zero-length string is false), but if it is an object (including an array) it will coerce to true.
If fields should be a string, then !fields is a sufficient predicate. If fields is an array, your best check might be:
if (!fields || fields.length === 0)