Question:

What is the correct usage with Promise.all() in Nodejs?

Problem

request module is a pretty popular module in Node.js and I am using it as single pretty simple:

request.get("url.com", function (error, response, body) {

console.log(body);

})

Pretty easy to handle with.

If I have 10 requests like this, imagine

var myReq1 = request.get("url.com", function (error, response, body) {

console.log(body);

})

var myReq2 = request.get("url.com", function (error, response, body) {

console.log(body);

})

var myReq3 = request.get("url.com", function (error, response, body) {

console.log(body);

})

var myReq4 = request.get("url.com", function (error, response, body) {

console.log(body);

})

And I want to have one callback for all when they all actually are done with their own callback function.

//body printed

Promise.all([myReq1, myReq2, myReq3, ....]).then((values) => {

console.log(values); //

});

Is this the correct usage with Promise.all() to achieve what I want ? Also,1 more question is, with request npm, does a promise resolve when the callback function to that specific request starts, or when it ends ?

To make it more clear,

request("url", (callback) => {

//Does the promise resolve here.

console.log('promise resolved now at start!')

....

console.log('promise will be resolved now after my logging')

//which one correct

})

Edit: This is a way with 1 promise I found in another thread, how to do with multiple?

function doRequest(url) {

return new Promise(function (resolve, reject) {

request(url, function (error, res, body) {

if (!error && res.statusCode === 200) {

resolve(body);

} else {

reject(error);

}

});

}

// Usage:

async function main() {

try {

let response = await doRequest(url);

console.log(response); // `response` will be whatever you passed to `resolve()` at the top

} catch (error) {

console.error(error); // `error` will be whatever you passed to `reject()` at the top

}

main();

Solution

Is this the correct usage with Promise.all() to achieve what I want ?
Yes, Promise.all() is the correct usage for your requirement. But "request" module won't return any promise. Wrap the response of "request module" with promise and use Promise.all(). >https://www.javascripttutorial.net/es6/javascript-promise-all/
does a promise resolve when the callback function to that specific request starts, or when it ends ?
Callback function will start only when the promise is resolved.

This is a way with 1 promise i found in another thread, how to do with multiple?
For multiple urls also you can do it in the same way.

Note: Nesting too many callbacks will lead to “callback hell” or “pyramid of doom.” Which is not recommended. Please use async and await. >https://javascript.info/async-await

// Function that do the api calls.

function doRequest(url) {

return new Promise(function (resolve, reject) {

request(url, function (error, res, body) {

if (!error && res.statusCode === 200) {

resolve(body);

} else {

reject(error);

}

});

}

To do the multiple API call, we can call the above "doRequest" function in 2 ways. Based on our use case.

// Usage: If we want to hit multiple APIs one after the other we can follow the below method.

async function main() {

try {

let response1 = await doRequest(url1); // url1

let response2 = await doRequest(url2); // url2

let response3 = await doRequest(url3); // url3

console.log("Got response for all API call"); // `response` will be whatever we passed to `resolve()` at the top

} catch (error) {

console.error(error); // `error` will be whatever you passed to `reject()` at the top

}

main();

// Usage: If we want to hit multiple api's in parallel and wait till the response to comes back. we can follow the below method.

async function main() {

let promise1 = doRequest(url1);

let promise2 = doRequest(url2);

let promise3 = doRequest(url3);

Promise.all([promise1, promise2, promise3]).then((values) => {

console.log(values); // resolved values of all promises. This value will be logged only when all the promises are resolved.

}).catch((error) => {

console.log(error); // rejected value of the promise.

});

}

Answered by: >Sat21343

Credit: >StackOverflow

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Nisha Patel

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