按顺序运行Q承诺并将值传递给下一个承诺

Question

I simplified my problem to easily give an illustration. I would like to implement the function sequential :

var Q = require('q');

sequential = function(list) {
    last_element = 0;
    results = [];
    list.forEach(function(element) {
        doSomething(element, last_element).then(function (result) {
            results.push(result);
        });
    })
    return results;
}

doSomething = function(element, last_element) {
    var defer = Q.defer();
    some_result_from_api = "this is supposed to be an async operation"
    result = element * last_element * some_result_from_api;
    defer.resolve(result);
    return defer.promise;
}

Obviously, that loop will not guarantee that last_element and results will be up to date as another doSomething promise runs. Is there an ideal way to do this using q?

Answer 1

Not sure why you are using Q with nodejs unless the node version you are using is seriously old.

Also can't make sense of what it is your code is supposed to do but since you have a list you can either map it to promises and process everything in parallel or reduce the list and process everything in series (one after the other):

 sequential = function(list) { //in parallel: // return Promise.all( // //last_element is 0 and never changes so I made it 0 // list.map(doSomething(element, 0)) // ); //serial; one after the other: return list.reduce( (p,item)=> p.then( results=> //last_element is 0 and never changes so I made it 0 doSomething(item, 0) .then( result=> results.concat([result]) ) ), Promise.resolve([]) ); } doSomething = async function(element, last_element) { const some_result_from_api = await "this is supposed to be an async operation" return [element, last_element, some_result_from_api]; } sequential([1,2,3]) .then( results=> console.log("Got results:",results) );