My learnings on TypeScript

never

• won’t allow the code to return undefined

• void function can return undefined

function functionThrow(): never {
    throw new Error("This function return never");
}

function outputMessage(message: string) {
    if (typeof message === "string") {
        console.log(message);
    } else {
        let invalid = message; // never type
        console.error(invalid);
    }
}

unknown

• better any

• any will compile successfully

• force you to use the type or type assertion

let variable2: unknown;
variable2 = "It is a string";
console.log(variable2.substr(0,2)) // Does not compile here

string literal

type Direction = "north" | "south" | "east" | "west";
let myDirection:Direction = "north";
// let yourDirection:Direction = "no-where"; // Does not compile

casting

const unknownType: unknown = "123"
    const cast1: number = <number>unknownType;
    const cast2: number = unknownType as number;