Basic Types
In TypeScript, a type annotation signals what values a variable can store. TypeScript is pretty smart. Many times they aren't even needed. This pages goes through the basics of type annotations so you know how to identify and use them yourself.
TIP
The type names String, Number, and Boolean (starting with capital letters) are legal, but refer to some special built-in types that will very rarely appear in your code. Always use string, number, or boolean for types.
string
ts
let s: string;
// runs
s = "dog"
// throws error
s = 4
boolean
ts
let isTrue: boolean = false;
number
int and float are both just number in typescript.
ts
let age: number = 30;
any
You can use any as a catch-all type. Pass anything you want and you won't get type check errors. Here's a code example from the official TypeScript docs.
ts
let obj: any = { x: 0 };
// None of the following lines of code will throw compiler errors.
// Using `any` disables all further type checking, and it is assumed
// you know the environment better than TypeScript.
obj.foo();
obj();
obj.bar = 100;
obj = "hello";
const n: number = obj;
array
ts
let n: number[];
n = [1,2,3];
let l: string[] = ['one', "two", "three"];
function
You can add type annotations to parameters.
ts
// param1 is a string
function myFunc(param1: string) {
console.log(param1)
}
// if you try to pass anything but a string into param1 you get an error.
You can also add return type annotations
ts
// param1 is a string
function myFunc(param1): string {
return param1
}
// This function must return a string, or else TS throws an error.
Most of the time, typescript will actually infer the return type, which means this isn't necessary. But some projects add it to help document whats happening in the code.