Checking whether a parameter belongs to a type in Typescript

Question

How can I check whether a certain parameter belongs to a type I've defined in TypeScript? For example:

type myType = {n:number}
let par = {n:3}

I want to check whether x is of the type mtType. If I use typeof par the returned value of it is the string "object". Thanks.

Answer 1

Take a look at the type guards section here. It is possible with the following syntax:

function isMyType(arg: any): arg is myType {
    // you can replace the following expression with the logic 
    // that clearly defines that if an object can be myType
    return arg.n !== undefined && arg.n === parseInt(arg.n);
}

and later in the code

if (isMyType(objectToCheck)){
   // do something with the object as myType
}