Why typescript function overload adds undefined type?

Question

I'm expecting data argument to be string type not string | undefined in type === 'scheduled' case. Why this is happening? Is there a way to make it string type?

function foo(type: 'live'): string;
function foo(type: 'scheduled', data: string): string;
function foo(type: 'live' | 'scheduled', data?: string): string {

    switch (type) {
        case 'live':
            return '';
        case 'scheduled':
            return data; // expecting for this to be string type not string | undefined
    }
}

Playground Link

you set data?: string, so type of data is string | undefined. — BeHappy
– BeHappy, Commented Jun 23, 2020 at 8:34
Inside the function only the last type definition is used, so the type of type is 'live' | 'scheduled' and the type of data is string | undefined. Given that you have additional information, you could do e.g. return data as string. — jonrsharpe
– jonrsharpe, Commented Jun 23, 2020 at 8:35

Nino Filiu · Accepted Answer · 2020-06-23 08:35:43Z

1

It's weird indeed, but you can use discriminated unions which work better:

function foo(t: { type: 'live' } | { type: 'scheduled', data: string }): string {
    switch (t.type) {
        case 'live':
            return '';
        case 'scheduled':
            t.data;
    }
}

Playground Link

answered Jun 23, 2020 at 8:35

Nino Filiu

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1 Comment

zimex Over a year ago

This solution also has some gotchas to it. type can't be deconstructed if you want to use it in switch statement Playground Link

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