Sending post request in for loop

Question

I would like to send post requests in loop. If i send for example 2 requests in a row only the last request really made the callback.

What am i do wrong?

Answer 1

Use a closure. Let me show you a simple example

// JavaScript on Client-Side
window.onload = function() {
    var f = (function() {
        for (i = 0; i < 3; i++) {
            (function(i){
                var xhr = new XMLHttpRequest();
                var url = "closure.php?data=" + i;
                xhr.open("GET", url, true);
                xhr.onreadystatechange = function () {
                    if (xhr.readyState == 4 && xhr.status == 200) {
                        console.log(xhr.responseText); // 0, 1, 2 
                    }
                };
                xhr.send();
            })(i);
        }
    })();
};

// Server-Side (PHP in this case)

In your case... wrap the asynchronous call/function with a closure

for (var i=0; i< this.selectedAvailableUnits.length; i++) {

    (function(i) {    // <--- the catch

        var unit = this.selectedAvailableUnits[i];
        var unitId = unit.unitId;
        var url = '/incident/' + currentIncidentId + '/assignUnit/' + unitId
        $http.post(url).then(function(response) {
            // DOING SOMETHING
        }, function(error) {
            alert(error);
        });

    })(i);    // <---- (the i variable might be omitted if it's not needed)

}

---

The section below is not directly related to the question but rather to the comments related to this answer.

---

The example submitted on jsFiddle mentioned in the comments and shown below is buggy and as such it doesn't prove anything.

It's true that this snippet, even not using a closure, yields 'Hello Kitty' three times; actually, if you replace console.log() method with the an alert() one you will see that it yields 'Hello Kitty' six, nine or even twelve times. So, what the hell is going one ;) how it's possible to get the alert window popping up six, nine or twelve times within a loop of three iterations?!

// your example (a)                                   // my comments 
//
var f = (function() {
    for (i = 0; i < 3; i++) {
        //(function(){                                // this way you can't have multiple scopes 
            var xhr = new XMLHttpRequest();
            var url = "closure.php?data=your-data";   // use /echo/html/ for testing on jsfiddle.net
            xhr.open("GET", url, true);               // use POST for testing on jsfiddle.net 
            xhr.onreadystatechange = function () {    // this way you might catch all readyStage property values
                callback();                           // this way the callback function will be called several times
            };
            xhr.send();
        //})();
    }
})();

var callback = function() {
    console.log("Hello Kitty"); // or use alert("Hello Kitty");
};

Output:

GET http://fiddle.jshell.net/_display/closure.php?data=your-data 404 (NOT FOUND) 
(9) Hello Kitty

As you could see, we've got an error and nine 'Hello Kitty' outputs in a row :) Before I change the function above let's see two important thing

First

onreadystatechange event stores a function or a reference to be called automatically each time the readyState property changes while status property holds the status of the XMLHttpRequest object.

readyState property possible values

• 0: request not initialized
• 1: server connection established
• 2: request received
• 3: processing request
• 4: request finished and response is ready

status property possible values

• 200: OK
• 404: Page not found

Second

As I said in the comments, jsfiddle.net isn't reliable for testing asynchronous snippets without some changes. In other words the GET method should be changed to POST and the url property must be changed to this link /echo/html/ (for more options take a look at jsFiddle documentation)

Now, let's change the example from the above (and follow the comments within the code)

// corrected example (b)
//
var f = (function() {
    for (i = 0; i < 3; i++) {
        //(function(i){                                              // uncomment this line for the 3rd output                               
            var xhr = new XMLHttpRequest();
            var url = "/echo/html";
            var data = "data";
            xhr.open("POST", url, true);
            xhr.onreadystatechange = function () {
                //if (xhr.readyState == 4 && xhr.status == 200) {    // uncomment this line for the 4th output
                    callback(i, xhr.readyState);                     // uncomment this line for the 4th output
                //}
            };
            xhr.send(data);
        //})(i);                                                     // uncomment this line for the 3rd output
    }
})();

var callback = function(i, s) {
    console.log("i=" + i + " - readyState=" + s + " - Hello Kitty");
};

1st output: // six outputs

(4) i=3 - readyState=1 - Hello Kitty    // four outputs related to readyState value 'server connection established'
    i=3 - readyState=2 - Hello Kitty    // related to readyState value 'request received'
    i=3 - readyState=4 - Hello Kitty    // related to readyState value 'request finished and response is ready'

2nd output: // six outputs

(2) i=3 - readyState=1 - Hello Kitty    // two outputs related to readyState value 'server connection established'
    i=3 - readyState=2 - Hello Kitty    // related to readyState value 'request received'
(3) i=3 - readyState=4 - Hello Kitty    // three outputs related to readyState value 'request finished and response is ready'

Without any changes made in example (b), we've got two different outputs. As you can see, different outputs for different readyState property values has been yield. But the value of i remained the same.

3rd output: // after uncommenting the lines for the 3rd output showned above in the example (b)

i=0 - readyState=2 - Hello Kitty        // related to readyState value 'request received'
i=0 - readyState=4 - Hello Kitty        // related to readyState value 'request finished and response is ready'
i=1 - readyState=2 - Hello Kitty        // ...
i=1 - readyState=4 - Hello Kitty        // ... 
i=2 - readyState=2 - Hello Kitty
i=2 - readyState=4 - Hello Kitty

So, after uncommenting the function which holds i as an argument, we see that the value of i has been saved. But this is still incorrect since there are six outputs and we need only three. As we don't need all the values of readyState or status property of the XMLHttpRequest object, let's uncomment the two lines needed for the fourth output

4th output: // after uncommenting the lines for the 4rd output showned above in the example (b) - finally three outputs

i=0 - readyState=4 - Hello Kitty
i=1 - readyState=4 - Hello Kitty
i=2 - readyState=4 - Hello Kitty 

Finally, this should be the correct version of the snippet and this is what we need.

Another almighty, omnipotent mechanism (as I figuratively said before) would be the bind() function which I don't prefer since it's slower than a closure.