I'm using Typescript with es2017 as the pilation target, using Javascript's new async / await.

I currently have the following code to fetch some data from a TS service class (simplified):

class MyClass {

    public static async initialize(): Promise<void> {
        const data = await this.getItems();
        // do unrelated initialization stuff
        return new Promise<void>(() => {});
    }

    private static async getItems(): Promise<Item[]> {
        return await Service.fetchData();
    }
}

class Service {
    public static async fetchData(): Promise<Item[]> {
        // Performs an XHR and returns a Promise.
    }
}

This works, but it would be a lot cleaner if MyClass::initialize() did not return anything, rather than returning new Promise<void>(() => {}); However, this seems to be impossible, as any method/function that uses await has to be marked async, and any method/function that is marked as async has to return a promise.

Is there any way around this, or is there something I'm fundamentally not grasping?

I'm using Typescript with es2017 as the pilation target, using Javascript's new async / await.

I currently have the following code to fetch some data from a TS service class (simplified):

class MyClass {

    public static async initialize(): Promise<void> {
        const data = await this.getItems();
        // do unrelated initialization stuff
        return new Promise<void>(() => {});
    }

    private static async getItems(): Promise<Item[]> {
        return await Service.fetchData();
    }
}

class Service {
    public static async fetchData(): Promise<Item[]> {
        // Performs an XHR and returns a Promise.
    }
}

This works, but it would be a lot cleaner if MyClass::initialize() did not return anything, rather than returning new Promise<void>(() => {}); However, this seems to be impossible, as any method/function that uses await has to be marked async, and any method/function that is marked as async has to return a promise.

Is there any way around this, or is there something I'm fundamentally not grasping?

• javascript
• typescript
• asynchronous
• async-await

Share Improve this question Follow asked Dec 30, 2017 at 22:39 NiekNiek 1,61911 gold badge1818 silver badges3030 bronze badges 5

• "has to return a promise" does not apply to js. JavaScript async functions return a promise automatically, The function's return value (not necessarily a promise) resolves the promise returned when calling the function. Is Typescript different? – traktor Commented Dec 30, 2017 at 22:57
• You should not write a class that has only static methods. Use an object literal instead. – Bergi Commented Dec 30, 2017 at 23:39
• Have a look at toVoid from my answer here – Bergi Commented Dec 30, 2017 at 23:42
• @Bergi why is that? – Niek Commented Dec 31, 2017 at 13:48
• 1 @Niek A class should only be used for when objects need to be instantiated. It's inefficient (and confusing) otherwise. Object literals are just simpler. – Bergi Commented Dec 31, 2017 at 15:27

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1 Answer 1

Sorted by: Reset to default 7

Async functions in TypeScript do need to be declared to return promises, but you don't actually need to return a promise from the function. You can just return the type of the promise value from the async function and it will be taken care of that it is wrapped in a promise.

So, for async functions that return a Promise<void>, you can just have an empty return or no return at all.

class Item
{
}

class MyClass 
{

    public static async VoidAsyncWithReturn(): Promise<void>
    {
        return;
    }

    public static async VoidAsyncWithoutReturn(): Promise<void>
    {
    }

    private static async AsyncReturningValue(): Promise<Item[]> 
    {
        var result: Item[] = new Array();
        return result;
    }

    private static async AsyncReturningPromise(): Promise<Item[]> 
    {
        var result: Promise<Item[]> = new Promise<Item[]>(() => { return new Array() });
        return result;
    }
}