JavaScript/TypeScript Asynchronous Filtering on AsyncIterable<T>

Question

JavaScript/TypeScript Asynchronous Filtering on AsyncIterable<T>

I am currently working with an AsyncIterable variable and I am trying to apply a filter on it. Take a look at the example below (pseudo code) -

class SomeClass {
    private SOME_CONST= "ABCDE";

    private async someFunction(): Promise<string> {
        const items: AsyncIterable<string> = await someLibrary.getAsyncItems(); // this library's function returns type of AsyncIterable<T>

        return items.filter(item => item === this.SOME_CONST);
    }
}

I am encountering an error when trying to filter on AsyncIterable - "Property 'filter' does not exist on type 'AsyncIterable'

javascript arrays typescript asynchronous

Answer 1

Answer №1

Filtering a synchronous iterator is not possible, as it needs to be converted into an array first.

Currently, there is no built-in method similar to Array.from() for AsyncIterable (see pending proposal), but you can achieve it by:

async function toArray<T>(asyncIterator: AsyncIterable<T>) {
  const arr = [];
  for await (const i of asyncIterator) arr.push(i);
  return arr;
}

After that, you can apply the filter like so:

declare const items: AsyncIterable<string>;

async function foo() {
  (await toArray(items)).filter(() => true)
}

Check out the TypeScript Playground for a demo.

Answer 2

Filtering a synchronous iterator is not possible, as it needs to be converted into an array first.

Currently, there is no built-in method similar to Array.from() for AsyncIterable (see pending proposal), but you can achieve it by:

async function toArray<T>(asyncIterator: AsyncIterable<T>) {
  const arr = [];
  for await (const i of asyncIterator) arr.push(i);
  return arr;
}

After that, you can apply the filter like so:

declare const items: AsyncIterable<string>;

async function foo() {
  (await toArray(items)).filter(() => true)
}

Check out the TypeScript Playground for a demo.

Answer 3

Answer №2

Breaking down the straightforward method:

export async function* filterAsyncIterable<T>(
    iterable: AsyncIterable<T>,
    predicate: (t: T) => boolean,
): AsyncIterable<T> {
    for await (const item of iterable) {
        if (predicate(item)) yield item;
    }
}

In a similar fashion, you can create mapAsyncIterable, among others. Additionally, you may require auxiliary functions such as asyncIterableToArray and asyncIterableFirst (for obtaining the initial element).

Subsequently, your code will transform into:

class SomeClass {
    private SOME_CONST= "ABCDE";

    private async someFunction(): AsyncIterable<string> {
        return filterAsyncIterable(
            someLibrary.getAsyncItems(), 
            item => item === this.SOME_CONST,
        );
    }
}

or potentially:

class SomeClass {
    private SOME_CONST= "ABCDE";

    private async someFunction(): Promise<string[]> {
        return asyncIterableToArray(filterAsyncIterable(
            someLibrary.getAsyncItems(), 
            item => item === this.SOME_CONST,
        ));
    }
}

Answer 4

Breaking down the straightforward method:

export async function* filterAsyncIterable<T>(
    iterable: AsyncIterable<T>,
    predicate: (t: T) => boolean,
): AsyncIterable<T> {
    for await (const item of iterable) {
        if (predicate(item)) yield item;
    }
}

In a similar fashion, you can create mapAsyncIterable, among others. Additionally, you may require auxiliary functions such as asyncIterableToArray and asyncIterableFirst (for obtaining the initial element).

Subsequently, your code will transform into:

class SomeClass {
    private SOME_CONST= "ABCDE";

    private async someFunction(): AsyncIterable<string> {
        return filterAsyncIterable(
            someLibrary.getAsyncItems(), 
            item => item === this.SOME_CONST,
        );
    }
}

or potentially:

class SomeClass {
    private SOME_CONST= "ABCDE";

    private async someFunction(): Promise<string[]> {
        return asyncIterableToArray(filterAsyncIterable(
            someLibrary.getAsyncItems(), 
            item => item === this.SOME_CONST,
        ));
    }
}

Answer 5

Answer №3

To solve this issue, you can easily create a new AsyncIterator by wrapping the original one using a generator function.

Here's a TypeScript example I put together:

export function filterAsyncIterator<T>(origin: AsyncIterableIterator<T>, predicate: (element: T) => boolean): AsyncIterableIterator<T> {
    return (async function* () {
        for await (const each of origin) {
            if(predicate(each)){
                yield each;
            }
        }
    })();
}

It's important to note that AsyncIterator is not a real type, but rather an interface that objects can adhere to by implementing a function named [Symbol.asyncIterator].

Answer 6

To solve this issue, you can easily create a new AsyncIterator by wrapping the original one using a generator function.

Here's a TypeScript example I put together:

export function filterAsyncIterator<T>(origin: AsyncIterableIterator<T>, predicate: (element: T) => boolean): AsyncIterableIterator<T> {
    return (async function* () {
        for await (const each of origin) {
            if(predicate(each)){
                yield each;
            }
        }
    })();
}

It's important to note that AsyncIterator is not a real type, but rather an interface that objects can adhere to by implementing a function named [Symbol.asyncIterator].

JavaScript/TypeScript Asynchronous Filtering on AsyncIterable<T>

Answer №1

Answer №2

Answer №3

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