Get typescript generic type as string value

Question

I'm trying to get the name type of a type given in a generic function.

This is for a nodeJS app.

I would like to do something like this:

static Get<T>(): string {
        return typeof T;
    }

But this exemple results as an error: "'T' only refers to a type, but is being used as a value here."

I would like "string" as a result if I call:

let strType: string = Get<string>();

The answer you've accepted gives you just a string whereas my answer gives you the exact literal string type e.g. number, object etc — Nurbol Alpysbayev
– Nurbol Alpysbayev, Commented Jul 1, 2019 at 13:34
The answer I've accepted tells that: "Since generics are not available at run time ", that is te answer of my question. Your answer is useful to bypass my problem. Thanks for that :) — GeJN
– GeJN, Commented Jul 1, 2019 at 14:36

Nurbol Alpysbayev · Accepted Answer · 2019-07-01 13:52:50Z

5

You can adapt this type from the TS Handbook:


type TypeName<T> =
    T extends string ? "string" :
    T extends number ? "number" :
    T extends boolean ? "boolean" :
    T extends undefined ? "undefined" :
    "object";

class Foo {

    static Get<T>(value: T): TypeName<T> {
        return typeof value;
    }
}

Foo.Get(123) // "number"
Foo.Get("str") // "string"

edited Jul 1, 2019 at 13:52

answered Jul 1, 2019 at 13:28

Nurbol Alpysbayev

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Comments

Murat Karagöz · Accepted Answer · 2019-07-01 13:20:17Z

2

Since generics are not available at run time there is no way to call typeof on that. However you can do it like this

function Get<T>(type: T): string {
        return typeof type;
    }

let strType: string = Get<number>(1);
strType: string = Get<string>("1");

answered Jul 1, 2019 at 13:20

Murat Karagöz

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4 Comments

Ric Over a year ago

or just simply Get(1), Get("1") (type inference)

Murat Karagöz Over a year ago

@Ric Sure, at that point you can also simply call typeof 1.

Ric Over a year ago

Yep absolutely, not sure what the use case of this actually is, but this is the answer

Aluan Haddad Over a year ago

You've explained the misconception correctly and concisely, but you should probably replace T with a non generic type string | number | boolean | null | undefined in your solution. Otherwise, the function may perpetuate the misconception in the OP.

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