Using a generator to call an API multiple times and only resolve when all requests are finished? - node.js

Question

I'm making a simple NodeJS app and I'm refactoring it out of my callback hell.
I've realised generators could be used but I'm struggling to grasp exactly how to use them.
Here's the basic flow of my function (I'm using the request-promise module):
// Iterate through keys to get values for
Object.keys(sourceData).forEach(function(key){
makeRequest(key);
})
makeRequest is a function that basically does this (it's incomplete):
// Make Request
function makeRequest(key) {
rp(apiEndpoint)
.then((data) => {
staticDictionary[key] = data.value;
})
}
I want to synchronously make a call to the endpoint, wait until it's finished getting the data, then move on to the next key in the loop using generators.
Can someone help?

Answer 1

You can sequence your requests one after the other, using .reduce() and promises.
// Make Request
function makeRequest(key) {
// use the key to construct the apiEndpoint here
return rp(apiEndpoint).then((data) => {
return data.value;
});
}
Object.keys(sourceData).reduce(function(p, key){
p = p.then(function(dictionary) {
return makeRequest(key).then(function(data) {
dictionary[key] = data;
return dictionary;
});
}
}, Promise.resolve({})).then(function(dictionary) {
// dictonary contains all your keys here
});
But, since none of your requests depend upon the prior requests, you could also run them all in parallel:
// Make Request
function makeRequest(key) {
// use the key to construct the apiEndpoint here
return rp(apiEndpoint).then((data) => {
return data.value;
});
}
// Iterate through keys to get values for
var staticDictionary = {};
Promise.all(Object.keys(sourceData).map(function(key){
return makeRequest(key).then(function(data) {
staticDictionary[key] = data;
});
})).then(function() {
// staticDictionary is fully available here
});
Using the Bluebird promise library, you could use Promise.props() which takes an object with promises as values and returns an object with the same keys, but resolved values as values (you pass in a map object and get back a map object):
// Make Request
function makeRequest(key) {
// use the key to construct the apiEndpoint here
return rp(apiEndpoint).then((data) => {
return data.value;
});
}
var promiseMap = {};
Object.keys(sourceData).forEach(function(key) {
promiseMap[key] = makeRequest(key);
})
return Promise.props(promiseMap).then(function(dictionary) {
// dictionary is available here
});

Answer 2

You can use Array.reduce function to do so.
Object.keys(sourceData).reduce(function(results, key){
makeRequest(key)
.then(function(data) {
results[key] = data;
});
}, {});
You can take a look at Bluebird, it has a lot of features related to what you are trying to acomplish.

Answer 3

Checkout co
co(function*(){
// Iterate through keys to get values for
for(const key in Object.keys(sourceData)){
// yield the promise
yield makeRequest(key);
}
});
// Make Request
function makeRequest(key) {
// return the promise
return rp(apiEndpoint)
.then((data) => {
staticDictionary[key] = data.value;
});
}
A better way is figuring out how does the ES6 generator work and probably do the things by yourself. Although co is a good library. :)