Passing parameters to a callback

Question

I'm trying to figure out how to pass my callback additional parameters. So far I have the following code.

$("#refine_mp_type").change(function(){
    var mp_type = $(this).val();
    var country_id = $('#refine_country').val();
    var dropdown = $('#refine_grade').find('select');
    getMPTypeGrades(country_id, mp_type, populateGradesDropdown);
});

function getMPTypeGrades(country_id, mp_type, callback){
    display_wait_dialog ("Please Wait", "Loading grades...");
    $.ajax(
    {
        //do ajax
    })
    .done(function(data){
        callback(data);
    })
    .fail(//do something)
    .always(//do something);
}

populateGradesDropdown(data){
    //populate the dropdown with the returned data
}

My thoughts so far on how to achieve this are:

I could set the dropdown variable to global scope and access it that way(Will pollute global scope).
I could pass the dropdown element to getMPTypeGrades function and then pass it to the callback function. (I want to keep getMPTypeGrades as generic as possible. I don't know if I'll always be passing a callback that accepts a JQuery element.)

What is the best method for passing the JQuery element and the Ajax response as parameters for my callback?

Maxx · Accepted Answer · 2016-08-03 13:57:50Z

2

You can bind dropdown to callback function:

$("#refine_mp_type").change(function(){
    var mp_type = $(this).val();
    var country_id = $('#refine_country').val();
    var dropdown = $('#refine_grade').find('select');
    getMPTypeGrades(country_id, mp_type, populateGradesDropdown.bind(null, dropdown));
});

Then

populateGradesDropdown(dropdown, data){
    //populate the dropdown with the returned data
}

answered Aug 3, 2016 at 13:57

Maxx

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2 Comments

Mouser Over a year ago

when you set the dropdown as the first argument of the bind, you can access it in populateGradesDropdown with the this keyword.

Maxx Over a year ago

Yep, but populateGradesDropdown might be universal function and may use its context. But you are right.

springrolls · Accepted Answer · 2016-08-04 06:17:45Z

2

Try to use jQuery Deferred and jQuery when. Basically you create a deferred object with a pending state and return it to the caller. jQuery when executes the callback function only when the deferred object has been resolved or rejected. When your ajax request is finished, .resolve or .reject will be called. This will trigger the callback method in the caller.

I haven't tested this code, but it should work more or less.

$("#refine_mp_type").change(function() {
    var mp_type = $(this).val();
    var country_id = $('#refine_country').val();
    var dropdown = $('#refine_grade').find('select');
    $.when(getMPTypeGrades(country_id, mp_type)).then(function(result) {
       //populate dropdown
    });
});

function getMPTypeGrades(country_id, mp_type) {
    var def = $.Deferred();   
    display_wait_dialog ("Please Wait", "Loading grades...");
    $.ajax(
    {
        //do ajax
    })
    .done(function(data) {
        def.resolve(data);
    })
    .fail(function(err) {
        def.reject(err);
    })
    .always(function() {
        hide_wait_dialog();
    });
    return def;
}

edited Aug 4, 2016 at 6:17

answered Aug 3, 2016 at 13:59

springrolls

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1 Comment

Mouser Over a year ago

linking is good, but give a little explanation about the deffered.

paddybasi · Accepted Answer · 2016-08-03 14:08:13Z

Rather than passing a callback (which can create a 'pyramid' code of callbacks), you should just return the deferred ajax object in getMPTypeGrades. You can then just use the standard .done()/.then() functions within the change handler:

$("#refine_mp_type").change(function(){
    var mp_type = $(this).val();
    var country_id = $('#refine_country').val();
    var dropdown = $('#refine_grade').find('select');
    getMPTypeGrades(country_id, mp_type).then(populateGradesDropdown);
});

function getMPTypeGrades(country_id, mp_type){
    display_wait_dialog ("Please Wait", "Loading grades...");
    return $.ajax(
    {
        //do ajax
    })
    .fail(//do something)
    .always(//do something);
}

populateGradesDropdown(data){
    //populate the dropdown with the returned data
}

Joe Enos · Accepted Answer · 2016-08-03 14:08:34Z

0

You can reference the original dropdown variable in the callback function body:

var dropdown = /*whatever*/;
getMPTypeGrades(country_id, mp_type, function(data) {
    populateGradesDropdown(data, dropdown);
});

$.ajax({
}).done(function(data){
    callback(data);
})

function populateGradesDropdown(data, dropdown){
    alert("data=" + data);
    alert("dropdown=" + dropdown);
}

answered Aug 3, 2016 at 14:08

Joe Enos

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