What is a Callback Function

A Callback Function for text(), html(), and val()

All of the three jQuery methods above: text(), html(), and val(), also come with a callback function. The callback function has two parameters: the index of the current element in the list of elements selected and the original (old) value. You then return the string you wish to use as the new value from the function.
The following example demonstrates text() and html() with a callback function:

Example

$("#btn1").click(function(){
    $("#test1").text(function(i, origText){
        return "Old text: " + origText + " New text: Hello world!
        (index: " + i + ")";
    });
});

$("#btn2").click(function(){
    $("#test2").html(function(i, origText){
        return "Old html: " + origText + " New html: Hello <b>world!</b>
        (index: " + i + ")";
    });
});