How to Get a Variable Type in Typescript?

Question

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
}

Accepted Answer

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

Answer 2

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
}

Answer 3

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();
    }
}

Answer 4

I have checked a variable if it is a boolean or not as below

console.log(isBoolean(this.myVariable));

Similarly we have

isNumber(this.myVariable);
isString(this.myvariable);

and so on.

Answer 5

I suspect you can adjust your approach a little and use something along the lines of the example here:

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

Answer 6

Must Read

Type guards in typescript

To determine the type of a variable after a conditional statement you can use type guards. A type guard in typescript is the following:

An expression which allows you to narrow down the type of something within a conditional block.

In other words it is an expression within a conditional block from where the typescript compiler has enough information to narrow down the type. The type will be more specific within the block of the type guard because the compiler has inferred more information about the type.

How to Get a Variable Type in Typescript?

8 Answers

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Must Read

Type guards in typescript

Sophia Al-Mansoor