I am a bit confused as to why my ajax call doesnt return a result. I thought a method defined as async automatically returns a promise. What am I doing wrong?
async AjaxCall(filePath) {
let xhttp = new XMLHttpRequest();
xhttp.open('POST', filePath, true);
xhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
xhttp.send();
xhttp.onreadystatechange = function() {
if (xhttp.readyState === 4 && xhttp.status === 200) {
return xhttp.responseText;
}
}
}
async function Test() {
var result = await AjaxCall("file.php");
alert(result);
}
Test();
async/await is (really useful) syntactic sugar for creating and consuming promises. Your AjaxCall function implicitly creates a promise, but then also implicitly resolves it immediately with the value undefined because you never await anything, and the only return isn't directly in AjaxCall but is instead in the onreadystatechange callback.
You can wrap a promise around XHR, but you don't have to: fetch already does:
async function Test() {
var result = await fetch("file.php");
if (result.ok) {
alert(await result.text());
}
}
But if you want to do it yourself, you'll need to explicitly create and consume a promise rather than using async on AjaxCall:
function AjaxCall(filePath) {
return new Promise((resolve, reject) => {
let xhttp = new XMLHttpRequest();
xhttp.open('POST', filePath, true);
xhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
xhttp.send();
xhttp.onreadystatechange = function() {
if (xhttp.readyState === 4) {
if (xhttp.status === 200) {
resolve(xhttp.responseText);
} else {
reject(); // Probably including some rejection reason
}
}
};
});
}
The problem is that you aren't actually returning any data from your function. You are returning data inside the onreadystatechange function but that is just lost and never used. Take a look here specifically this piece of code:
function makeRequest(method, url) {
return new Promise(function (resolve, reject) {
let xhr = new XMLHttpRequest();
xhr.open(method, url);
xhr.onload = function () {
if (this.status >= 200 && this.status < 300) {
resolve(xhr.response);
} else {
reject({
status: this.status,
statusText: xhr.statusText
});
}
};
xhr.onerror = function () {
reject({
status: this.status,
statusText: xhr.statusText
});
};
xhr.send();
});
}
You will notice that it has wrapped the entire function in a promise and then you can use the standard async/await functionality when calling it. Async/Await is really just a wrapper around the existing promise functionality.
Related
Imagine this function:
function myMainFunction() {
doSomeInitialStuff();
// more stuff..
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
if (xhr.readyState == XMLHttpRequest.DONE) {
// Now that we know we received the result, we can do the heavy lifting here
if (xhr.status == 200) {
console.log("ready 200");
let result = JSON.parse(xhr.responseText);
doStuff(result);
// and much more stuff..
} else {
console.log("error", xhr.status);
return undefined;
}
}
};
xhr.open("GET", "http://example.com", true);
xhr.send(null);
}
This works fine, but it is impossible to test, and this function has become a monster.
So I'd like to refactor it, by separating all the different parts in their own unique functions.
The problem is, I do not know how to extract the XHR part and still keep it working.
I cannot use Promises nor asnyc/await and have to stick to using plain XHR.
What I'd normally do is to create a seperate async function for the ajax call (or the xhr in this case). Simply await it's result and go from there. Easy to separate. But I do not have the luxury of await or anything this time.
What I am trying to get at is something like this
function refactoredMyMainFunction() {
doSomeInitialStuff();
// more stuff..
let result = xhrFunction();
doStuff(result); // result would be undefined here, since I cannot wait for the xhr request to finish.
}
You can implement a callback-based API:
function myMainFunction() {
doSomeInitialStuff();
// more stuff..
xhrFunction(doStuff);
}
function xhrFunction(cb) {
var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
if (xhr.readyState == XMLHttpRequest.DONE) {
// Now that we know we received the result, we can do the heavy lifting here
if (xhr.status == 200) {
console.log("ready 200");
let result = JSON.parse(xhr.responseText);
cb(result);
// and much more stuff..
} else {
console.log("error", xhr.status);
return undefined;
}
}
};
xhr.open("GET", "http://example.com", true);
xhr.send(null);
}
is this possible? I want to write an ajax function, that I do not want to duplicate it. Pass it different parameter which are locations to different files. Then use the promise to make them into one object. I would possible use the spread operator. is this possible.
var myFuncCalls = 0;
let promiseAjax = new Promise (function ( resolve,reject) {
//possibly use a for look to grab the number of times the loadDoc was called then call the same function and send it to may be an array?
function loadDoc(location) {
myFuncCalls++;
console.log("loadDoc was called :" + myFuncCalls);
var xyz = new XMLHttpRequest();
xyz.onreadystatechange = function () {
if (this.readyState == 4 && this.status == 200) {
//console.log(this.responseText)
resolve(this.responseText);
}
};
xyz.open("GET", location, true);
xyz.send();
}
loadDoc("/_js/someitems.json");
loadDoc("/_js/someMoreItems.json");
})
// then grab all that stuff and make one single object using spread operators
promiseAjax.then(function (fromResolve){
// JSON.parse(fromResolve);
var newObj = JSON.parse(fromResolve);
console.log(newObj);
})
with Promise.all and Object.assign,
function loadDoc(location) {
return new Promise((resolve, reject) => {
var xyz = new XMLHttpRequest();
xyz.onreadystatechange = () => {
if (this.readyState == 4 && this.status == 200) {
resolve(JSON.parse(this.responseText));
} else {
// resolving with empty object to avoid breaking other fetch if one failed
resolve({});
}
};
xyz.open("GET", location, true);
xyz.send();
});
}
const loadDocs = (paths) => Promise.all(paths.map(path => loadDoc(path))
.then(results => {
// combine all result into single object
return Object.assign({}, ...results);
}));
// example
loadDocs([
"/_js/someitems.json",
"/_js/someMoreItems.json"
]).then(function(finalCombinedObject) {
// other logic here
});
Use Promise.all() to get the two calls together and so what ever you want with the array of the data you resolved.
function loadDoc(location) {
return new Promise (function ( resolve,reject) {
var xyz = new XMLHttpRequest();
xyz.onreadystatechange = function () {
if (this.readyState == 4 && this.status == 200) {
resolve(this.responseText);
}
};
xyz.open("GET", location, true);
xyz.send();
});
}
const urls = ["/_js/someitems.json", "/_js/someMoreItems.json"]
Promise.all(urls.map(url=>loadDoc(url))).then(responses =>
console.log(responses);
)
I think the easiest thing would be to define async functions, which return promises and can be easily passed around and reused.
You can do something like:
async function loadFile(file) {
...
return {...fileJSON};
}
async function loadFiles() {
const file1JSON = await loadFile('file1');
const file2JSON = await loadFile('file2');
return {...file1JSON, ...file2JSON};
}
loadFiles().then((combinedJSON) => {
...
})
These functions can take arguments and be reused like any other function.
This kind of behavior can archived with Promise.all Promise.all white the use of async+await async and the use of more state of the art calls (fetch) makes the code looks cleaner
async function loadAll(docs) {
return Promise.all(docs.map(async doc => {
const result = await fetch('http://example.com/movies.json');
return result.json();
}));
}
(async function() {
const responses = await loadAll(["/_js/someitems.json", "/_js/someMoreItems.json"]);
console.log(responses);
})();
Note: await can only be used from an async function.
Note2: the code is untested
Yes, youcan send the URL, any parameters, even the type of AJAX call (POST, GET, etc), to the method, then use it to build the call. This way, you can reuse the same method to do anything and everything you need to do from your client with a "simple" method call.
All code in this Answer is copied from the below link.
https://medium.com/front-end-weekly/ajax-async-callback-promise-e98f8074ebd7
function makeAjaxCall(url, methodType)
{
var promiseObj = new Promise(function(resolve, reject)
{
var xhr = new XMLHttpRequest();
xhr.open(methodType, url, true);
xhr.send();
xhr.onreadystatechange = function()
{
if (xhr.readyState === 4)
{
if (xhr.status === 200)
{
console.log("xhr done successfully");
var resp = xhr.responseText;
var respJson = JSON.parse(resp);
resolve(respJson);
}
else
{
reject(xhr.status);
console.log("xhr failed");
}
}
else {console.log('xhr processing going on');}
}
console.log("request sent succesfully");
});
return promiseObj;
}
enter code here
document.getElementById('userDetails').addEventListener('click', function()
{
// git hub url to get btford details
var userId = document.getElementById("userId").value;
var URL = "https://api.github.com/users/"+userId;
makeAjaxCall(URL, "GET").then(processUserDetailsResponse, errorHandler);
});
You can even send it the callback method. I also send it a method to use for errors.
function makeAjaxCall(url, methodType, callback)
{
$.ajax(
{
url : url,
method : methodType,
dataType : "json",
success : callback,
error : function (reason, xhr){
console.log("error in processing your request", reason);
}
});
}
// git hub url to get btford details
var URL = "https://api.github.com/users/btford";
makeAjaxCall(URL, "GET", function(respJson)
{
document.getElementById("userid").innerHTML = respJson.login;
document.getElementById("name").innerHTML = respJson.name;
document.getElementById("company").innerHTML = respJson.company;
document.getElementById("blog").innerHTML = respJson.blog;
document.getElementById("location").innerHTML = respJson.location;
});
I'm trying to create an excel add-in using Javascript that requires asynchronous functions return a JS promise to Excel and that the promise is resolved with the final value using the callback function. I am new to promises and have spent hours reading and testing this out with no success, and was hoping someone could help me understand what I'm doing wrong. Below is the code:
function TEST(num1) {
return new Promise(function (resolve) {
var corsproxy = "https://cors-anywhere.herokuapp.com/"
var apiurl = "https://randomapi.com/testapi"
var data = getData(corsproxy+apiurl).then(function(result){
console.log ("here it comes")
console.log(result.meta.status) /// This returns "Success"
return (result.meta.status) /// I need this to be resolved
})
console.log("last")
resolve(data)
})
};
/// Get JSON
function getData(url) {
return new Promise(function (resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.responseType = 'json';
xhr.onload = function () {
try {
if (xhr.status === 200) {
resolve(xhr.response);
}
else if (xhr.status !== 200) {
reject({
error: 'Request failed. ' + xhr.response
});
}
} catch (e) {
reject({
error: e
});
}
};
xhr.send();
});
}
The second function getData is working as intended and returning a JS Object. What I'm trying to accomplish with the TEST function is:
Create a new promise - this needs to be resolved/returned as "Success"
Call the API data with getData(temporarily running through a proxy to bypass CORS erros)
Extract the meta.status value ("Success")
I've tried a number of different things but the current code write "last" and resolves an undefined data before the getData function completes. Changing the "return (result.meta.status)" to "resolve (result.meta.status)" also doesn't help.
Any assistance with what I'm doing wrong would be greatly appreciated.
function TEST(num1) {
var corsproxy = "https://cors-anywhere.herokuapp.com/"
var apiurl = "https://randomapi.com/testapi"
return getData(corsproxy+apiurl)
}
TEST(valofnum1).then(function(result){
console.log ("here it comes")
console.log(result.meta.status) /// This returns "Success"
return (result.meta.status) /// Needs to be resolved
})
that's how you chain promises. you don't resolve a promise within another promise,
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
You can use async/await since ES6 that solves a lot of this headache by simplifying the chain process into using await statements when letting promises resolve. I've updated your code block to use it, take a look:
async function TEST(num1) {
return new Promise(function (resolve) {
var corsproxy = "https://cors-anywhere.herokuapp.com/"
var apiurl = "https://randomapi.com/testapi"
var result = await getData(corsproxy+apiurl);
resolve(result.meta.status)
})
};
/// Get JSON
function getData(url) {
return new Promise(function (resolve, reject) {
var xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.responseType = 'json';
xhr.onload = function () {
try {
if (xhr.status === 200) {
resolve(xhr.response);
}
else if (xhr.status !== 200) {
reject({
error: 'Request failed. ' + xhr.response
});
}
} catch (e) {
reject({
error: e
});
}
};
xhr.send();
});
}
Changes made:
TESTis now an async function.
Rather chaining the resolved promise from getData and resolving in TEST, you simply await the response from getData and then resolve it.
I've spent last few hours trying to make it work but it just doesn't for no apparent reason. I have all the required packages and settings. I get no errors, async and await just doesn't wait.
I use Webpack to require the polyfill files that Babel adds, e.g babel-runtime/regenerator.
Code:
async function getData() {
let data = await ajaxCall();
console.log(data);
}
function ajaxCall() {
let url = 'https://maps.googleapis.com/maps/api/geocode/json?address=london';
let xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == XMLHttpRequest.DONE) {
if(xmlhttp.status == 200) {
console.log(JSON.parse(xmlhttp.response));
return JSON.parse(xmlhttp.response);
}
}
}
xmlhttp.open('GET', url, true);
xmlhttp.send();
}
getData();
// It logs "undefined" and then ajax response AFTER
.babelrc:
{
"presets": ["es2015", "stage-0"],
"plugins": ["transform-runtime"]
}
Does anyone have any idea what could be wrong?
In order for async and await to work you need to return something from your awaited call that the JS runtime can use to know when to continue the function that is awaiting the result. The "type" that is required for async / await interop is called Promise.
Your ajaxCall returns undefined, which doesn't tell the JS runtime anything, so it doesn't await because there is nothing to wait for. If you want to make this work, simply return a Promise from ajaxCall and resolve it when your ajax request is fulfilled.
At the simplest:
function ajaxCall() {
let url = 'https://maps.googleapis.com/maps/api/geocode/json?address=london';
// The new `window.fetch` API returns a promise for you
return fetch(url).then(response => response.json());
}
or using XMLHttpRequest:
function ajaxCall() {
let url = 'https://maps.googleapis.com/maps/api/geocode/json?address=london';
return new Promise((resolve, reject) => {
let xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = () => {
if (xmlhttp.readyState == XMLHttpRequest.DONE) {
if (xmlhttp.status == 200) {
// resolve instead of return inside of a Promise closure
resolve(JSON.parse(xmlhttp.response));
} else {
// reject instead of throw
// (will throw the error at the `await` expression.)
reject(Error(`Received status code ${xmlhttp.status}`));
}
}
}
xmlhttp.open('GET', url, true);
xmlhttp.send();
});
}
You need to return a promise from the ajaxCall().
Code should look something like that:
function ajaxCall() {
return new Promise((resolve, reject) => {
let url = 'https://maps.googleapis.com/maps/api/geocode/json?address=london';
let xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == XMLHttpRequest.DONE) {
if(xmlhttp.status == 200) {
console.log(JSON.parse(xmlhttp.response));
resolve(JSON.parse(xmlhttp.response));
}
}
// handle error: reject(error);
}
xmlhttp.open('GET', url, true);
xmlhttp.send();
})
}
then:
async function getData() {
try {
let data = await ajaxCall();
console.log(data);
} catch (e) {
// do something with error
}
}
Note that I used es6 arrow functions
I am using a javascript function that calls another javascript function (zConvertEmplidtoRowid) that uses an ajax call that runs a query to return data in a variable (rowid). My problem is I don't know how to return the data to the original function.
Here is a snippet of the original function calling the ajax function (zConvertEmplidtoRowid)
var rowid = zConvertEmplidtoRowid(emplid);
//the alert should show what the query via ajax returned
alert(rowid);
zEmployeePortrait(emplid, ajaxlink);
}
And here is the ajax function...I imagine somewhere in here I need to place the return, but I've never used ajax before, so I'm not sure.
function zConvertEmplidtoRowid(emplid, ajaxlink, callback) {
if (typeof XMLHttpRequest == 'undefined') {
XMLHttpRequest = function() {
try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) {}
try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {}
throw new Error('This browser does not support XMLHttpRequest or XMLHTTP.');
};
}
var request = new XMLHttpRequest();
request.onreadystatechange = function() {
if (request.readyState == 4 && request.status == 200) {
var rowid = request.responseText;
callback(rowid);
}
}
var ajax_link = ajax_link + "?emplid=" + emplid;
request.open('GET', ajax_link);
request.send();
}
As #epascarello pointed out, the ajax call is asynchronous and the code you have written is expecting the call to return in a synchronous way.
You have two options:
1) Make the ajax call synchronous (I highly recommend not to take this route).
2) Pass a callback function as a parameter to the function making the ajax call and then invoke the callback function once the call returns.
e.g:
function zConvertEmplidtoRowid(emplid, ajaxlink, callback) { //Added a callback function parameter
if (typeof XMLHttpRequest == 'undefined') {
XMLHttpRequest = function() {
try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch(e) {}
try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) {}
try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {}
throw new Error('This browser does not support XMLHttpRequest or XMLHTTP.');
};
}
var request = new XMLHttpRequest();
request.onreadystatechange = function() {
if (request.readyState == 4 && request.status == 200) {
var rowid = request.responseText;
//now you invoke the callback passing the rowid as argument
callback(rowid);
}
}
var ajax_link = ajax_link + "?emplid=" + emplid;
request.open('GET', ajax_link);
request.send();
}
zConvertEmplidtoRowid(emplid, ajaxlink, function(rowId) {
alert(rowId);
zEmployeePortrait(emplid, ajaxlink);
});
As epascarello has implied in his comment, you need to make the javascript call synchronously in order to get a return value...
I tend to use jquery to assist with the call, but a quick google suggests you can do it your way by changing:
request.open('GET', ajax_link);
to:
request.open('GET', ajax_link, false);
and the response is then accessible through:
request.responseText
taken from here: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests