Confirm whether the Iterator type is the same as the AsyncIterator type

Question

Confirm whether the Iterator type is the same as the AsyncIterator type

Is there a clever JavaScript technique to differentiate between Iterator and AsyncIterator without initiating the iteration process?

I'm attempting to create a type checker like this:

function isAsyncIterator<T>(i: Iterator<T> | AsyncIterator<T>): boolean {
    // should return:
    //  - true, if i is an asynchronous iterator
    //  - false, if i is a synchronous iterator
}

I am aware that using next would provide this information, but I need to determine it without actually iterating through.

Although my example was in TypeScript, I require a strict run-time check for this matter.

javascript typescript iterator

Answer 1

Answer №1

Ascertaining whether an object is an iterator is not straightforward. Iterators are determined by a protocol rather than a tangible property, making it challenging to determine without actually iterating through the object. This challenge is due to the halting problem, as explained in this post on Stack Overflow: What is the technical definition of a Javascript iterable and how do you test for it?.

However, there are reliable methods that can detect most properly implemented iterators and all built-in iterators:

One indication that an object is an iterator is if it is iterable (meaning it can be used in for … of loops) and will delegate to itself for iteration. Here's a sample code snippet to check for iterators:

function isIterator(obj) {
    if (Object(obj) !== obj) return false;
    const method = obj[Symbol.iterator];
    if (typeof method != 'function') return false;
    const iter = method.call(obj);
    return iter === obj;
}
function isAsyncIterator(obj) {
    if (Object(obj) !== obj) return false;
    const method = obj[Symbol.asyncIterator];
    if (typeof method != 'function') return false;
    const aIter = method.call(obj);
    return aIter === obj;
}

Another approach is to check if the object inherits from the builtin %IteratorPrototype%. While accessing and testing these objects may be cumbersome until the introduction of the iterator helpers proposal, it is still possible. Here's a code snippet for this method:

const GeneratorPrototype = Object.getPrototypeOf(function*() {}).prototype;
const IteratorPrototype = Object.getPrototypeOf(GeneratorPrototype);
const AsyncGeneratorPrototype = Object.getPrototypeOf(async function*() {}).prototype;
const AsyncIteratorPrototype = Object.getPrototypeOf(AsyncGeneratorPrototype);

function isIterator(obj) {
    return IteratorPrototype.isPrototypeOf(obj);
}
function isAsyncIterator(obj) {
    return AsyncIteratorPrototype.isPrototypeOf(obj);
}

(Note: This method does not work with objects from other realms; hence, the first approach is recommended.)

Answer 2

Ascertaining whether an object is an iterator is not straightforward. Iterators are determined by a protocol rather than a tangible property, making it challenging to determine without actually iterating through the object. This challenge is due to the halting problem, as explained in this post on Stack Overflow: What is the technical definition of a Javascript iterable and how do you test for it?.

However, there are reliable methods that can detect most properly implemented iterators and all built-in iterators:

One indication that an object is an iterator is if it is iterable (meaning it can be used in for … of loops) and will delegate to itself for iteration. Here's a sample code snippet to check for iterators:

function isIterator(obj) {
    if (Object(obj) !== obj) return false;
    const method = obj[Symbol.iterator];
    if (typeof method != 'function') return false;
    const iter = method.call(obj);
    return iter === obj;
}
function isAsyncIterator(obj) {
    if (Object(obj) !== obj) return false;
    const method = obj[Symbol.asyncIterator];
    if (typeof method != 'function') return false;
    const aIter = method.call(obj);
    return aIter === obj;
}

Another approach is to check if the object inherits from the builtin %IteratorPrototype%. While accessing and testing these objects may be cumbersome until the introduction of the iterator helpers proposal, it is still possible. Here's a code snippet for this method:

const GeneratorPrototype = Object.getPrototypeOf(function*() {}).prototype;
const IteratorPrototype = Object.getPrototypeOf(GeneratorPrototype);
const AsyncGeneratorPrototype = Object.getPrototypeOf(async function*() {}).prototype;
const AsyncIteratorPrototype = Object.getPrototypeOf(AsyncGeneratorPrototype);

function isIterator(obj) {
    return IteratorPrototype.isPrototypeOf(obj);
}
function isAsyncIterator(obj) {
    return AsyncIteratorPrototype.isPrototypeOf(obj);
}

(Note: This method does not work with objects from other realms; hence, the first approach is recommended.)

Answer 3

Answer №2

Exploring various methods to overcome an obstacle in my writing, I discovered a unique solution:

To address this issue, consider implementing a typed iterator with the sync flag included, as outlined below:

interface TypedIterator<T, TReturn = any, TNext = undefined> {
    next(...args: [] | [TNext]): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    return?(value?: TReturn | PromiseLike<TReturn>): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    throw?(e?: any): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    sync: boolean;
}

Furthermore, a conversion utility can be provided for seamless integration:

function toTypedIterator<T>(i: Iterator<T> | AsyncIterator<T>): TypedIterator<T> {
    let started = false;
    const v = i.next() as any;
    const sync = !v || typeof v.then !== 'function';
    return {
        ...i,
        sync,
        next() {
            if (started) {
                return i.next();
            }
            started = true;
            return v;
        }
    };
}

To put this solution to the test, consider using the following example:

const data = [1, 2, 3, 4, 5];

const i = toTypedIterator(data[Symbol.iterator]());
console.log('sync:', i.sync);

do {
    const a = i.next();
    if (a.done) {
        break;
    }
    console.log(a);
} while (true);

The resulting output showcases the success of this approach:

sync: true
{ value: 1, done: false }
{ value: 2, done: false }
{ value: 3, done: false }
{ value: 4, done: false }
{ value: 5, done: false }

Answer 4

Exploring various methods to overcome an obstacle in my writing, I discovered a unique solution:

To address this issue, consider implementing a typed iterator with the sync flag included, as outlined below:

interface TypedIterator<T, TReturn = any, TNext = undefined> {
    next(...args: [] | [TNext]): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    return?(value?: TReturn | PromiseLike<TReturn>): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    throw?(e?: any): IteratorResult<T, TReturn> | Promise<IteratorResult<T, TReturn>>;

    sync: boolean;
}

Furthermore, a conversion utility can be provided for seamless integration:

function toTypedIterator<T>(i: Iterator<T> | AsyncIterator<T>): TypedIterator<T> {
    let started = false;
    const v = i.next() as any;
    const sync = !v || typeof v.then !== 'function';
    return {
        ...i,
        sync,
        next() {
            if (started) {
                return i.next();
            }
            started = true;
            return v;
        }
    };
}

To put this solution to the test, consider using the following example:

const data = [1, 2, 3, 4, 5];

const i = toTypedIterator(data[Symbol.iterator]());
console.log('sync:', i.sync);

do {
    const a = i.next();
    if (a.done) {
        break;
    }
    console.log(a);
} while (true);

The resulting output showcases the success of this approach:

sync: true
{ value: 1, done: false }
{ value: 2, done: false }
{ value: 3, done: false }
{ value: 4, done: false }
{ value: 5, done: false }

Confirm whether the Iterator type is the same as the AsyncIterator type

Answer №1

Answer №2

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