Typescript notation async function as type/interface attribute

Question

I'm having trouble with the syntax for an async (arrow) function as a type/interface attribute. I've already done a bit of research but didn't found much except this https://github.com/Microsoft/TypeScript/issues/22035 It's not 100% accurate to my case but better than nothing. And still doesn't work for me...

Here is about the code I have in the declaration (simplified) :

export type MyType = {
    att1: string;
    funct: async (param1: string, param2: string) => Promise<string[]>;
};

And where I use it (example, not my actual code) :

import { MyType } from "./MyType";

const myObject: MyType = {
    att1: "First attribute";
    funct: async (param1: string, param2: string): Promise<string[]> => {
        // Code below is an example, in my case it's a database querying
        // But anyway, it's something async
        return new Promise((resolve, reject): void => {
            setTimeout((): void => {
                resolve(["string1", "string2"]);
            }, 5000);
        });
    };
};

The TypeScript transpiler says it cannot find the name "async". How do I tell the function "funct" in MyType is async? I need to use await in it...

Titian Cernicova-Dragomir · Accepted Answer · 2019-01-26 21:00:08Z

27

The answer is simple, you don't tell the compiler the function is async on the interface. async is an implementation detail, it allows you to use await inside the body of the function. The user of the interface does not care how you implement your function only what it returns and what parameters need to be passed in.

What you really want is to tell the compiler that the function does not return string[] but rather a Promise<string[]>. This means that the caller can't access the result directly but needs to either use then on the returned promise or use await to get the result.

export type MyType = {
    att1: string;
    funct:  (param1: string, param2: string) => Promise<string[]>;
};

edited Jan 26, 2019 at 21:00

answered Jan 26, 2019 at 20:53

Titian Cernicova-Dragomir

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5 Comments

Ascor8522 Over a year ago

The return type in the type definition is something I forgot to change... I just didn't know you don't have to add the async keyword in the interface. Any way, thank you very much.

MikeMajara Over a year ago

What if I want to force the function to be async. I explicitly want the user of the interface to make the method an async one?

Titian Cernicova-Dragomir Over a year ago

@MikeMajara no way to force this. From a consumer standpoint there really is no way to tell the difference between an async method and a method returning a promise.

Bruno Negrão Zica Over a year ago

how can I create a class that implements that interface and having the async method?

konrad Over a year ago

@BrunoNegrãoZica eg.: class MyClass implements MyType { funct (param1: string, param2: string) {.return Promise ((res, rej) => { // something async here leading to res(someArrayOfStrings) or rej(error) }. }

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