Javascript XMLHttpRequest in for loop

Question

I know this question has been asked before, but I tried to apply the answers with no results.

I'm trying to do multiple requests on the same domain with a for loop but it's working only for the last record of my array. When I try with only one request it works fine. I don't understand.

Here is the code I use :

var xhr = new XMLHttpRequest();
var idArray = ['1', '2', '3', '4', '5'];

for(var i = 0;i < idArray.length;i++) {
    xhr.open('PUT', 'https://www.domain.com/url/' + idArray[i]);
    xhr.setRequestHeader('Authorization', authorizationToken);
    xhr.send(null);
    var test = setInterval(function () {
        if(xhr.readyState != 4) {
            //someCode
        } else {
            clearInterval(test);
        }
    }, 1000);
    xhr.onreadystatechange = function () {
        if (xhr.readyState == 4) {
            if(xhr.status != 200) {
                //someCode
            }
        }
    }
}

I've also tried this but still no results:

var xhr = new XMLHttpRequest();
var idArray = ['1', '2', '3', '4', '5'];

for(var i = 0;i < idArray.length;i++) {
    (function(i) {
        xhr.open('PUT', 'https://www.domain.com/url/' + idArray[i]);
        xhr.setRequestHeader('Authorization', authorizationToken);
        xhr.send(null);
        var test = setInterval(function () {
            if(xhr.readyState != 4) {
                //someCode
            } else {
                clearInterval(test);
            }
        }, 1000);
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                if(xhr.status != 200) {
                    //someCode
                }
            }
        }
    })(i);
}

I'm not seeing what I'm doing wrong.

T.J. Crowder · Accepted Answer · 2024-02-25 15:22:46Z

3

Your second attempt is fairly close, but you need to create a separate XMLHttpRequest object for each request, within the inline-invoked function expression, see the relocated line with the *** comment:

var idArray = ['1', '2', '3', '4', '5'];

for(var i = 0;i < idArray.length;i++) {
    (function(i) {
        var xhr = new XMLHttpRequest(); // ***
        xhr.open('PUT', 'https://www.example.com/url/' + idArray[i]);
        xhr.setRequestHeader('Authorization', authorizationToken);
        xhr.send(null);
        var test = setInterval(function () {
            if(xhr.readyState != 4) {
                //someCode
            } else {
                clearInterval(test);
            }
        }, 1000);
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                if(xhr.status != 200) {
                    //someCode
                }
            }
        }
    })(i);
}

Technically, since you don't use i anywhere in the callbacks you're creating in there, you don't need to pass i in and take it as a parameter to the IIFE (but with that ES5-level syntax, you do need the IIFE so that you have separate xhrs).

In ES2015+, there's no need for the IIFE anymore, just use let and const:

const idArray = ["1", "2", "3", "4", "5"];

for (const id of idArray) {
    const xhr = new XMLHttpRequest();
    xhr.open("PUT", "https://www.example.com/url/" + id);
    xhr.setRequestHeader("Authorization", authorizationToken);
    xhr.send(null);
    const test = setInterval(() => {
        if (xhr.readyState !== 4) {
            //someCode
        } else {
            clearInterval(test);
        }
    }, 1000);
    xhr.onreadystatechange = () => {
        if (xhr.readyState == 4) {
            if (xhr.status !== 200) {
                //someCode
            }
        }
    };
}

These days I'd probably use fetch instead of XHR as well.

Side note: Not sure what the interval timer is for there, so I've left it, but your onreadystatechange handler will get called, no need to back it up with a timer.

edited Feb 25, 2024 at 15:22

answered Dec 13, 2016 at 15:26

T.J. Crowder

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2 Comments

Davide Andrea Over a year ago

For those who (like me) don't know: IIFE = (Immediately Invoked Function Expression)

T.J. Crowder Over a year ago

@DavideAndrea - Sorry, I really shouldn't have just used the acronym there. I've updated and modernized the answer. :-)

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