How to wait for a promise to resolve inside a map?

Question

I have a list of objects that I want to process. The object is passed to a promise function that does this and that and resolves back. The process could be instant or not, based on previously cached value. If there is already calculated value, it will resolve to it instantly. Else, it will calculate. Now the issue I am having is that the next object is passed to the promise before the first object's status is calcualted:

   let people = [ 
                {groupId: 1, name: 'Jessica Coleman', status: 'Unknown', id:1}
                {groupId: 1, name: 'Eric Tomson', status: 'Unknown', id:2}
                {groupId: 1, name: 'Samuel Bell', status: 'Unknown', id:3}

      ];

now I want to absolutely wait for the promise to resolve during loop even if the promise takes a minute to calculate on the very instance. All people with the same group have the same status. Hence, the promise checks if a group has already been calculated. If yes, returns it. Else, it calculates. and that's where the issue lies. Before Jessica 1 is finished, the other people are passed.

    people.map(function(person) {
   // return the promise to array
   this.calculatorService
    .getStatus(person)
    .then(function(res) {
      person.status = res;


    });
});

Answer 1

Array iterators like map or forEach don't work with promises because they don't know how to await a result. Use a simple for loop instead:

for (let person of people)
  person.status = await this.calculatorService.getStatus(person)

If you really want a "functional" way (and avoid explicit async/await), you can define a function similar to the bluebird's Promise.each:

Promise.each = function(ary, fn) {
    return ary.reduce((p, x) => p.then(() => fn(x)), Promise.resolve(null))
}

and apply it like this:

function setStatus(person) {
    return calculatorService
        .getStatus(person)
        .then(res => person.status = res);
}

Promise.each(people, setStatus).then(...)

Answer 2

Make it work synchronously with async/await. (for..of would be better suited than .map in this case btw).

for (let person of people) {
   person.status = await this.calculatorService.getStatus(person);
})