So lately I was working with a team member on an Angular Pipe. We had an Address Interface and wanted to only allow an argument that was property from the Adress Interface. But how do you define that?

It turns out very simple, we need to use the keyof operator!

Interface

First, we have to check the Address interface. Below is a more simplified version of what we use, but this is perfect for this example.

interface Address {
    streetname: string;
    housnumber: number;
    postal: string;
    city: string;
}

Use keyof operator

With the keyof operator, we can say that a parameter should be equal to the property of an interface.

function fakeFunc(param: keyof Address): void {
    console.log(param);
}

Now we can ensure that this function is only being used with a parameter that exists in the Address interface. Otherwise, you will get a TypeScript interface.

Validate the function

In the example you can see that the param ‘streetname’ is accepted but the parameter ‘random’ is giving the error “Argument of type '"random"' is not assignable to parameter of type 'keyof Address'.” because it’s not in the interface. (To see the compile error, you need to click the check it on CodeSandbox)

Thanks!

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