How can I use Typescript to model an object via type instead of an interface?

Question

I have the following JSON which I have no control over:

{
    "date": "2020-01-01T00:00:00",
    "a": 0,
    "b": 0,
    "c": 0
}

Instead of specifying an interface:

interface Foo {
    date: Date,
    "a": number,
    "b": number,
    "c": number
}

I want to be able to statically type the payload like so:

type Age = "a";
type Salary = "b";
type Height = "c"

type Foo = Record<"date", Date> | Record<Age | Salary | Height, number>;

Even though the compiler seems happy but when I try to get the date from the object I get the following error:

const foo = {
        "date": "2020-01-01T00:00:00",
        "a": 0,
        "b": 0,
        "c": 0
    } as Foo;

Element implicitly has 'any' type.

If I remove the Record<Age | Salary | Height, number> then the error goes away.

can you specify which element reported to implicitly has any type ? — Ahmad Shaarawi
– Ahmad Shaarawi, Commented Apr 13, 2020 at 11:52

Aleksey L. · Accepted Answer · 2020-04-13 12:34:03Z

1

You should use intersection instead of union:

type Foo = Record<"date", string> & Record<Age | Salary | Height, number>

** I've changed date type to string because you're assigning string value to this field

answered Apr 13, 2020 at 12:14

Aleksey L.

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