How can I get jQuery to perform a synchronous, rather than asynchronous, Ajax request?

How can I get jQuery to perform a synchronous, rather than asynchronous, Ajax request?

I have a JavaScript widget which provides standard extension points. One of them is the beforecreate function. It should return false to prevent an item from being created.
I’ve added an Ajax call into this function using jQuery:
beforecreate: function (node, targetNode, type, to) {
jQuery.get(‘http://example.com/catalog/create/’ + targetNode.id + ‘?name=’ + encode(to.inp[0].value),

function (result) {
if (result.isOk == false)
alert(result.message);
});
}

But I want to prevent my widget from creating the item, so I should return false in the mother-function, not in the callback. Is there a way to perform a synchronous AJAX request using jQuery or any other in-browser API?

Solutions/Answers:

Solution 1:

From the jQuery documentation: you specify the asynchronous option to be false to get a synchronous Ajax request. Then your callback can set some data before your mother function proceeds.

Here’s what your code would look like if changed as suggested:

beforecreate: function (node, targetNode, type, to) {
    jQuery.ajax({
        url: 'http://example.com/catalog/create/' + targetNode.id + '?name=' + encode(to.inp[0].value),
        success: function (result) {
            if (result.isOk == false) alert(result.message);
        },
        async: false
    });
}

Solution 2:

You can put the jQuery’s Ajax setup in synchronous mode by calling

jQuery.ajaxSetup({async:false});

And then perform your Ajax calls using jQuery.get( ... );

Then just turning it on again once

jQuery.ajaxSetup({async:true});

I guess it works out the same thing as suggested by @Adam, but it might be helpful to someone that does want to reconfigure their jQuery.get() or jQuery.post() to the more elaborate jQuery.ajax() syntax.

Solution 3:

Excellent solution! I noticed when I tried to implement it that if I returned a value in the success clause, it came back as undefined. I had to store it in a variable and return that variable. This is the method I came up with:

function getWhatever() {
  // strUrl is whatever URL you need to call
  var strUrl = "", strReturn = "";

  jQuery.ajax({
    url: strUrl,
    success: function(html) {
      strReturn = html;
    },
    async:false
  });

  return strReturn;
}

Solution 4:

All of these answers miss the point that doing an Ajax call with async:false will cause the browser to hang until the Ajax request completes. Using a flow control library will solve this problem without hanging up the browser. Here is an example with Frame.js:

beforecreate: function(node,targetNode,type,to) {

    Frame(function(next)){

        jQuery.get('http://example.com/catalog/create/', next);
    });

    Frame(function(next, response)){

        alert(response);
        next();
    });

    Frame.init();
}

Solution 5:

function getURL(url){
    return $.ajax({
        type: "GET",
        url: url,
        cache: false,
        async: false
    }).responseText;
}


//example use
var msg=getURL("message.php");
alert(msg);

Solution 6:

Keep in mind that if you’re doing a cross-domain Ajax call (by using JSONP) – you can’t do it synchronously, the async flag will be ignored by jQuery.

$.ajax({
    url: "testserver.php",
    dataType: 'jsonp', // jsonp
    async: false //IGNORED!!
});

For JSONP-calls you could use:

  1. Ajax-call to your own domain – and do the cross-domain call server-side
  2. Change your code to work asynchronously
  3. Use a “function sequencer” library like Frame.js (this answer)
  4. Block the UI instead of blocking the execution (this answer) (my favourite way)