Extending an interface by changing the object in an object array

Question

I have a base interface like:

interface base {
    property1: string
    property2: string;
    property3: [{
        something1: string;
        something2: string;
    }];
}

I would like to have an interface like:

property1: string
property2: string;
property3: [{
    something1: string;
    something2: string;
    something3: number;
}];
property4: number;

I was able to achieve the propert4 using extend like:

interface extended {
    property4: number
}

I also read up on how to extend a property and was doing it like this:

type extendedProperty = base['property3'];

interface extendedPropertyInterface extends extendedProperty {
    something3: number;
}
interface extended {
    property3: extendedPropertyInterface;
    property4: number;
}

This does not work. I see that extendedPropert is of the type

[{
    something1: string;
    something2: string;
}];

(Notice the Array brackets). How do I get what I want to achieve?

EDIT: I cannot change the base interface.

Generics might help here

Aleksey L.
– Aleksey L.

2017-09-17 10:59:52 +00:00
Commented Sep 17, 2017 at 10:59 — Aleksey L.
– Aleksey L., Commented Sep 17, 2017 at 10:59

Nitzan Tomer · Accepted Answer · 2017-09-17 11:13:29Z

1

First of all, this form: [type] declares a tuple and not an array, it should be: type[].

As for what you asked about, you can do this:

interface ArrayItem {
    something1: string;
    something2: string;
}

interface Base<T extends ArrayItem = ArrayItem> {
    property1: string
    property2: string;
    property3: T[];
}

interface ExtendedItem extends ArrayItem {
    something3: number;
}

interface Extended extends Base<ExtendedItem> {
    property4: number;
}

(code in playground)

Edit

As you cannot change base, this seems to work:

interface extended extends base {
    property3: [{
        something1: string;
        something2: string;
        something3: number;
    }];
    property4: number;
}

edited Sep 17, 2017 at 11:13

answered Sep 17, 2017 at 11:01

Nitzan Tomer

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5 Comments

Sayak Mukhopadhyay Over a year ago

Ah! I should have said this but I can't change the Base interface. Else, I would have done the same. I will make an edit to the question to that effect.

Nitzan Tomer Over a year ago

Check my revised answer

Aleksey L. Over a year ago

You can also reuse base type: property3: [base['property3'][0] & {something3: number}];

Sayak Mukhopadhyay Over a year ago

@AlekseyL.Awesome. That's exactly what I was looking for. Can you please explain what the & is doing here?

Nitzan Tomer Over a year ago

That's intersection types

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