Sequential execution of Promises using reduce()

promises_reduce.js

function asyncFunc(e) {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(e), e * 1000);
  });
}

const arr = [1, 2, 3];
let final = [];

function workMyCollection(arr) {
  return arr.reduce((promise, item) => {
    return promise
      .then((result) => {
        console.log(`item ${item}`);
        return asyncFunc(item).then(result => final.push(result));
      })
      .catch(console.error);
  }, Promise.resolve());
}

workMyCollection(arr)
  .then(() => console.log(`FINAL RESULT is ${final}`));

reducing example to nearest simplest form:

[1,2,3].reduce((promise,item) => {
	return promise.then(() => { 
		return new Promise((resolve, reject)=> {
			resolve(item);
		})
	})
},Promise.resolve())

await array.reduce(async (promise, item) => {
  await promise;
  return asyncFunction(item);
}, true);

I know the goal is to use reduce/be functional here but just to point out using async function and a loop to make sure it works the same way Promise.all does in the end with how it resolves

async function resolveSequentially(arr) {
  const results = [];
  for (const v of arr) {
    if (typeof v === 'function') {
      results.push(await v());
    } else {
      results.push(await v);
    }
  }
  return results;
}

or the reduce parallel

function resolveSequentially(arr) {
  let r = Promise.resolve();
  return Promise.all(
    arr.reduce((p, c) => {
      r = r.then(() => (typeof c === 'function' ? c() : c));
      p.push(r);
      return p;
    }, []),
  );
}

it's friday 4:50pm and this came in at the perfect time. thank you so much!!

Hey seriously. Echoing the above thanks!

Using reduce for this makes it harder than it has to be. It's not very easy on the eyes and mind.
I'd prefer something like this if await / async is available.

// functions is an array of functions that return a promise.
async function runInSequence(functions) {
  const results = [];

  for (const fn of functions) {
    results.push(await fn());
  }

  return results;
}

And we can use it like this:

function promisedFunction(delay, value) {
  return new Promise(resolve => {
    setTimeout(() => resolve(value), delay);
  });
}

console.time("execution");
const results = await runInSequence([
  promisedFunction.bind(this, 1000, 1),
  promisedFunction.bind(this, 1000, 2),
  promisedFunction.bind(this, 1000, 3)
]);

console.timeEnd("execution"); // execution: 3000 ms (approx)
console.log(results); // [1, 2, 3]

Thanks. Helped me a lot to solution duplicate primary key constraint in save() method with Typeform. The parallel process was my issue and I changed my Promise.all( ) loop, for this fn, and it works great.

My pay:

class TasksService {
  static async execute(context, tasks) {
    let results = [];
     await tasks.reduce((promise, task) => {
      return promise
        .then((result) => {
          return TasksService.asyncFunc(task).then(r => {
            results.push(r.bind(context)())
          });
        })
    }, Promise.resolve());
    return results;
  }

  static asyncFunc(f) {
    return new Promise((resolve, reject) => {
      let task = f;
      let time = 0;
      if(typeof(task) !== 'function') {
        task = f.task;
        time = f.time;
      }
      setTimeout(() => resolve(task), time);
    });
  }
}

module.exports = TasksService;