This post explains the differences very well. They are the same in TypeScript as in JavaScript.
As for what you should use: You may define that on your own. You may use either, just be aware of the differences and it might make sense to be consistent.
The TypeScript coding style guide for the TypeScript source code (not an official "how to use TypeScript" guide) states that you should always use undefined and not null: Typescript Project Styleguide.
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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?
This post explains the differences very well. They are the same in TypeScript as in JavaScript.
As for what you should use: You may define that on your own. You may use either, just be aware of the differences and it might make sense to be consistent.
The TypeScript coding style guide for the TypeScript source code (not an official "how to use TypeScript" guide) states that you should always use undefined and not null: Typescript Project Styleguide.
The value 'undefined' denotes that a variable has been declared, but hasn't been assigned any value. So, the value of the variable is 'undefined'.
On the other hand, 'null' refers to a non-existent object, which basically means 'empty' or 'nothing'.
You can manually assign the value 'undefined' to a variable, but that isn't recommended. So, 'null' is assigned to a variable to specify that the variable doesn't contain any value or is empty. But 'undefined' is used to check whether the variable has been assigned any value after declaration.
Using a juggling-check, you can test both null and undefined in one hit:
Copyif (x == null) {
If you use a strict-check, it will only be true for values set to null and won't evaluate as true for undefined variables:
Copyif (x === null) {
You can try this with various values using this example:
Copyvar a: number;
var b: number = null;
function check(x, name) {
if (x == null) {
console.log(name + ' == null');
}
if (x === null) {
console.log(name + ' === null');
}
if (typeof x === 'undefined') {
console.log(name + ' is undefined');
}
}
check(a, 'a');
check(b, 'b');
Output
"a == null"
"a is undefined"
"b == null"
"b === null"
Copyif( value ) {
}
will evaluate to true if value is not:
nullundefinedNaN- empty string
'' 0false
typescript includes javascript rules.