async-abstraction/tape

/**
 * Utility function to chain an async iterable of async iterables.
 *
 * @private
 * @param {AsyncIterable<AsyncIterable>} iterables - the input iterable of iterables.
 * @returns {AsyncIterable} The chain of the input iterables.
 */
export default function asyncIterableChain(iterables) {
    return new Chain(iterables);
}

export class Chain {
    constructor(iterables) {
        this.iterators = iterables[Symbol.asyncIterator]();
        this.iterator = null;
    }

    async next() {
        for (;;) {
            if (this.iterator === null) {
                // eslint-disable-next-line no-await-in-loop
                const {done, value} = await this.iterators.next();
                if (done) return {done: true};
                this.iterator = value[Symbol.asyncIterator]();
            }

            // eslint-disable-next-line no-await-in-loop
            const {done, value} = await this.iterator.next();
            if (done) {
                this.iterator = null;
            } else {
                return {done: false, value};
            }
        }
    }

    [Symbol.asyncIterator]() {
        return this;
    }
}