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
}
This question is related to
typescript
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.
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();
}
}
I suspect you can adjust your approach a little and use something along the lines of the example here:
https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards
function isFish(pet: Fish | Bird): pet is Fish {
return (pet as Fish).swim !== undefined;
}
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.
declare let abc: number | string;
// typeof abc === 'string' is a type guard
if (typeof abc === 'string') {
// abc: string
console.log('abc is a string here')
} else {
// abc: number, only option because the previous type guard removed the option of string
console.log('abc is a number here')
}
Besides the typeof
operator there are built in type guards like instanceof
, in
and even your own type guards.
Source: Stackoverflow.com