Resolve of promise happening before forEach loop finishes

Question

So, I am attempting to run a server testing function, using node and request. My goal is to run through an array of server objects, build an array of successful server pings, and return a single array with the best priority (hard coded into the server objects). Here is my code so far:

//brings in http script availability
var request = require('request');

//set of test servers
var serverSet = [
      {
        "url": "http://doesNotExist.askdex.co",
        "priority": 1
      },
      {
        "url": "http://tricon.co",
        "priority": 7
      },
      {
        "url": "http://offline.bing.co",
        "priority": 2
      },
      {
        "url": "http://google.com",
        "priority": 4
      },
      {
        "url": "http://yahoo.com",
        "priority": 6
      }
];

//inform user of process taking place
console.log("We are about to test some server requests for you, please stand by.");


function serverTest(url, priority) {
    //creates options to allow for request timeout
    var options = {
        url: url,
        timeout: 5000
    };
    return new Promise (
        function(resolve, reject) {
            request(options, function(err, res, body) {
                //if (err) {console.log("There was an error: " + err)};
                //test if server responds with a positive status
                if (res !== undefined) {
                    if (res.statusCode >= 200 && res.statusCode <= 299) {
                        //console.log("response from online server is " + res.statusCode);
                        resolve({"url": url, "priority": priority});
                    } else
                    if (res.statusCode >= 300 && res.statusCode <= 399) {
                        //console.log("response from redirected server is " + res.statusCode);
                        reject("The server is not working");
                    } else
                    if (res.statusCode >= 400 && res.statusCode <= 499) {
                        //console.log("response from not working server is " + res.statusCode);
                        reject("The server is broken");
                    }//==> end of inner if/else statement
                } else {
                    reject("Server is unresponsive");
                }//==> end of outer if/else statement
            });//==> end of get request
        }
    );
};

//call of function to run program
//var test0 = serverTest(serverSet[0].url, serverSet[0].priority).then(function(resolve){console.log("resolution is: "+ JSON.stringify(resolve))}).catch(function(reject){console.log(reject)});
//console.log("Test0: "+ test0);
//var test1 = serverTest(serverSet[1].url, serverSet[1].priority).then(function(resolve){console.log("resolution is: "+ JSON.stringify(resolve))}).catch(function(reject){console.log(reject)});
//console.log("Test1: "+ JSON.stringify(test1));
//var test2 = serverTest(serverSet[2].url, serverSet[2].priority).then(function(resolve){console.log("resolution is: "+ JSON.stringify(resolve))}).catch(function(reject){console.log(reject)});
//console.log("Test2: "+ JSON.stringify(test2));
//var test3 = serverTest(serverSet[3].url, serverSet[3].priority).then(function(resolve){console.log("resolution is: "+ JSON.stringify(resolve))}).catch(function(reject){console.log(reject)});
//console.log("Test3: "+ test3);
//var test4 = serverTest(serverSet[4].url, serverSet[4].priority).then(function(resolve){console.log("resolution is: "+ JSON.stringify(resolve))}).catch(function(reject){console.log(reject)});
//console.log("Test4: "+ JSON.stringify(test4));

function findServer(array) {
    var build = [];
    return new Promise (
        function(resolution, rejection) {
            array.forEach(function(server){
                serverTest(server.url, server.priority)
                    .then(function(resolve){
                        if (typeof resolve === "object") {
                            console.log("findServer resolve = " + JSON.stringify(resolve));
                            build.push(resolve);
                            console.log("build = " + JSON.stringify(build));
                            return build;
                        }
                    })
                    .catch(function(error){
                        //console.log("Error: " + error);
                    });
            });
                    resolution(build);
            // if (onlineServers.length == 0) {
            //  //console.log("No online servers.");
            //  reject('Error: No servers are online.');
            // }//==> end of if statement
        }
    )
};

findServer(serverSet).then(function(result) {
    console.log("The result is " + JSON.stringify(result));
    }).catch(function(error){
        console.log("The error is " + error);
    });
// function findServer(array) {
//  return new Promise (
//      function(resolution, rejection) {
//          var build = [];
//          array.forEach(function(server){
//              serverTest(server.url, server.priority).then(function(resolve){
//                      if (typeof resolve === "object") {
//                          console.log("findServer resolve = " + JSON.stringify(resolve));
//                          build.push(resolve);
//                          console.log("build = " + JSON.stringify(build));
//                          resolution(build);
//                      }
//                  }).catch(function(error){
//                      //console.log("Error: " + error);
//                  });
//          });
//          // if (onlineServers.length == 0) {
//          //  //console.log("No online servers.");
//          //  reject('Error: No servers are online.');
//          // }//==> end of if statement
//      }
//  )
// };




 // findServer(serverSet).then(function(result){
    //      console.log(result);
    //      console.log(onlineServers);
    //      //condition check and modification if more than one server response is successful
    //      if (result.length > 1) {
    //          result.sort(function(a,b){
    //              return a.priority - b.priority;
    //          })
    //          console.log(result[0].url + " has the lowest available priority.");
    //      } else {
    //          console.log(result[0].url + " has the lowest available priority.");
    //      }//==> end of if statement
    //  }).catch(function(error){
    //      console.log(error);
    //  });

The commented-out code contains numerous attempts to test and figure out other ways to make this work.

Answer 1

You'll want to use Promise.all() to wait until many Promises have resolved until continuing, but be aware of this caveat (emphasis mine) :

The Promise.all(iterable) method returns a promise that resolves when all of the promises in the iterable argument have resolved, or rejects with the reason of the first passed promise that rejects .

So in your case, you need to make sure that all the promises you pass to .all() will resolve. For this you can use Promise.catch() to handle rejections from your serverTest() function and continue gracefully.

The Promise returned by catch() is rejected if onRejected throws an error or returns a Promise which is itself rejected; otherwise, it is resolved.

 var serverSet = [{ "url": "http://doesNotExist.askdex.co", "priority": 1 }, { "url": "http://tricon.co", "priority": 7 }, { "url": "http://offline.bing.co", "priority": 2 }, { "url": "http://google.com", "priority": 4 }, { "url": "http://yahoo.com", "priority": 6 }] function serverTest(url, priority) { var options = { url: url, timeout: 5000 } return new Promise((resolve, reject) => { // Always resolve for testing purposes resolve({ "url": url, "priority": priority }) }) } function findServer(servers) { var build = [] var promises = servers.map(server => { return serverTest(server.url, server.priority) .then(resolve => { // Do your validation of resolve here build.push(resolve) }) .catch(error => { // By using catch, you ensure this promise chain will continue to // resolve as long as you don't throw an error here or return another // Promise that will reject console.log("Server " + server.url + " failed with : " + error) }) }) return Promise.all(promises).then(values => { // At this point you know that all your promises have resolved, and // the successful ones have added an element to 'build' return build }); }; findServer(serverSet).then(result => { console.log("result is : " + JSON.stringify(result)) }) 

Edit : using map() instead of forEach + push() , thanks for suggestions in comments.

Edit 2 : Added code for complete example, but modified serverTest so that it always resolves, in order to verify that findServer populates the build array correctly.