test01.js

(() => {
  class Parallel {
    constructor({ parallelJobs }) {
        this.parallelJobs = parallelJobs;
        this.jobsQueue = [];
        this.results = [];
    }
    _flatten(arrray) {
        return arrray.reduce((acc, value) => acc.concat(value), []);
    }
    _runParallel(jobs, done) {
        let countCollectedValues = 0;
        const values = [];

        function collectValues({ index, value }) {
            values[index] = value;
            countCollectedValues++;

            if (countCollectedValues === jobs.length) {
                done(values);
            }
        }

        jobs.forEach((job, index) => job((value) => collectValues({ index, value })));
    }
    _runEach(actions, done) {
        let countCollectedValues = 0;
        const values = [];

        function collectValues({ index, value }) {
            values[index] = value;
            countCollectedValues++;

            if (countCollectedValues === actions.length) {
                done(values);
            } else {
                doAction();
            }
        }

        function doAction() {
            const action = actions[countCollectedValues];
            action((value) => collectValues({ index: countCollectedValues, value }))
        }

        doAction();
    }
    _doWithLimit(actions, limit, done) {
        let actionsCopy = [...actions];
        const jobsBatches = [];

        while (actionsCopy.length) {
            const limitedActions = actionsCopy.slice(0, limit);

            jobsBatches.push((done) => {
                setTimeout(() => {
                    this._runParallel(limitedActions, done);
                }, 0)
            });

            actionsCopy = actionsCopy.slice(limit);
        }

        this._runEach(jobsBatches, done);
    }
    job(callback) {
        this.jobsQueue.push(callback);
        return this;
    }
    done(callback) {
        this._doWithLimit(this.jobsQueue, this.parallelJobs, (results) => {
            callback(this._flatten(results));
        });
    }
  }

const runner = new Parallel({
	parallelJobs: 2
});

let result = 'before/';

runner
	.job(step0)
	.job(step1)
	.job(step2)
	.job(step3)
	.job(step4)
	.done(onDone);


result += 'after/';

function step0(done) {
	console.log('step0');

	result += 'step0/';
	
	done('step0');
}

function step1(done) {
	console.log('step1');
	
	result += 'step1/';
	
	setTimeout(done, 3000, 'step1');
}

setTimeout(() => result += 'after0-1/', 2500);

function step2(done) {
	console.log('step2');

	result += 'step2/';

	setTimeout(done, 1500, 'step2');
}

function step3(done) {
	console.log('step3');

	result += 'step3/';

	setTimeout(done, 2000, 'step3');
}

setTimeout(() => result += 'after2-3/', 4500);

function step4(done) {
	console.log('step4');
	

	result += 'step4/';
	
	setTimeout(done, 500, 'step4');
}

setTimeout(() => result += 'after4/', 5500);

let isPassed = false;
function onDone(results) {
	console.log('onDone', results, result);

	console.assert(Array.isArray(results), 'expect result to be array');
	console.assert(results.length === 5, 'the results length must be 5');
	console.assert(results[0] === 'step0', 'Wrong answer 1');
	console.assert(results[1] === 'step1', 'Wrong answer 2');
	console.assert(results[2] === 'step2', 'Wrong answer 2');
	console.assert(results[3] === 'step3', 'Wrong answer 3');
	console.assert(results[4] === 'step4', 'Wrong answer 4');
	console.assert(result === 'before/after/step0/step1/after0-1/step2/step3/after2-3/step4/after4/', 'Wrong steps');
	console.log('Thanks, all works fine');
	
	isPassed = true;
}

setTimeout(() => {
	if(isPassed) return;

	console.error('Test is not done.');
}, 6000);
})();

test02.js

(() => {
class Parallel {
    constructor({ parallelJobs }) {
        this.parallelJobs = parallelJobs;
        this.jobsQueue = [];
        this.results = [];
    }
    async _runAsync(action) {
        return new Promise((resolve) => {
            setTimeout(() => {
                resolve(action());
            }, 0)
        });
    }
    async _runParallel(actions) {
        return Promise.all(
            actions.map((action) => new Promise(resolve => action(resolve)))
        );
    }
    async _doWithLimit(actions, limit) {
        let actionsCopy = [...actions];
        let results = [];
        const actionsBatches = [];

        while (actionsCopy.length) {
            const limitedActions = actionsCopy.slice(0, limit);
            actionsBatches.push(limitedActions);
            actionsCopy = actionsCopy.slice(limit);
        }

        for (const actions of actionsBatches) {
            const res = await this._runAsync(() => this._runParallel(actions));
            results = results.concat(res);
        }

        return results;
    }
    job(callback) {
        this.jobsQueue.push(callback);
        return this;
    }
    async done(callback) {
        const results = await this._doWithLimit(this.jobsQueue, this.parallelJobs);
        callback(results);
    }
}

const runner = new Parallel({
	parallelJobs: 2
});

let result = 'before/';

runner
	.job(step0)
	.job(step1)
	.job(step2)
	.job(step3)
	.job(step4)
	.done(onDone);


result += 'after/';

function step0(done) {
	console.log('step0');

	result += 'step0/';
	
	done('step0');
}

function step1(done) {
	console.log('step1');
	
	result += 'step1/';
	
	setTimeout(done, 3000, 'step1');
}

setTimeout(() => result += 'after0-1/', 2500);

function step2(done) {
	console.log('step2');

	result += 'step2/';

	setTimeout(done, 1500, 'step2');
}

function step3(done) {
	console.log('step3');

	result += 'step3/';

	setTimeout(done, 2000, 'step3');
}

setTimeout(() => result += 'after2-3/', 4500);

function step4(done) {
	console.log('step4');
	

	result += 'step4/';
	
	setTimeout(done, 500, 'step4');
}

setTimeout(() => result += 'after4/', 5500);

let isPassed = false;
function onDone(results) {
	console.log('onDone', results, result);

	console.assert(Array.isArray(results), 'expect result to be array');
	console.assert(results.length === 5, 'the results length must be 5');
	console.assert(results[0] === 'step0', 'Wrong answer 1');
	console.assert(results[1] === 'step1', 'Wrong answer 2');
	console.assert(results[2] === 'step2', 'Wrong answer 2');
	console.assert(results[3] === 'step3', 'Wrong answer 3');
	console.assert(results[4] === 'step4', 'Wrong answer 4');
	console.assert(result === 'before/after/step0/step1/after0-1/step2/step3/after2-3/step4/after4/', 'Wrong steps');
	console.log('Thanks, all works fine');
	
	isPassed = true;
}

setTimeout(() => {
	if(isPassed) return;

	console.error('Test is not done.');
}, 6000);
})();

test01 - первый вариант на коллбеках
test02 - второй вариант на промисах