How to get a variable type in Typescript?

I have a variable.

abc:number|string;

How can I check its type? I want to do something like below:

if (abc.type === "number") {
    // do something
}

For :

abc:number|string;

Use the JavaScript operator typeof:

if (typeof abc === "number") {
    // do something
}

TypeScript understands typeof 🌹

This is called a typeguard.

More

For classes you would use instanceof e.g.

class Foo {}
class Bar {} 

// Later
if (fooOrBar instanceof Foo){
  // TypeScript now knows that `fooOrBar` is `Foo`
}

There are also other type guards e.g. in etc https://basarat.gitbooks.io/typescript/content/docs/types/typeGuard.html

I'd like to add that TypeGuards only work on strings or numbers, if you want to compare an object use instanceof

if(task.id instanceof UUID) {
  //foo
}

The other answers are right, but when you're dealing with interfaces you cannot use typeof or instanceof because interfaces don't get compiled to javascript.

Instead you can use a typecast + function check typeguard to check your variable:

interface Car {
    drive(): void;
    honkTheHorn(): void;
}

interface Bike {
    drive(): void;
    ringTheBell(): void;
}

function start(vehicle: Bike | Car ) {
    vehicle.drive();

    // typecast and check if the function exists
    if ((<Bike>vehicle).ringTheBell) {
        const bike = (<Bike>vehicle);
        bike.ringTheBell();
    } else {
        const car = (<Car>vehicle);
        car.honkTheHorn();
    }
}

And this is the compiled JavaScript in ES2017:

function start(vehicle) {
    vehicle.drive();
    if (vehicle.ringTheBell) {
        const bike = vehicle;
        bike.ringTheBell();
    }
    else {
        const car = vehicle;
        car.honkTheHorn();
    }
}