CODEWITHSUNDEEP
typescripttypeof
Zev Spitz
Zev Spitz

Reputation: 15317

Type guard of function type

I have a function with a union type between string and a function type. How can I write a type guard to narrow to that function type? (The following doesn't work.)

interface Callback {
    (args: any[]): void
}

var fn = function(arg: string | Callback) {
    if (typeof arg == 'Callback') {
        arg.call([]);
    }
}

Upvotes: 1

Views: 1716

Answers (1)

David Sherret
David Sherret

Reputation: 106640

typeof won't ever return 'Callback'. In this scenario it will work to do an instanceof Function check:

interface Callback {
    (args: any[]): void
}

var fn = function(arg: string | Callback) {
    if (arg instanceof Function) {
        arg.call([]);
    }
};

Alternatively this also works: 

if (typeof arg === "function") {
    arg.call([]);
}

Upvotes: 5

Related Questions