How to know order occurrence of selected/highlighted text jquery

Question

I think I understand what you are trying to achieve, I have an idea though it's not solid but maybe this can give you a head start.

Get the highlighted text and find all similar text in the body and wrap it with <span>. Then get the <span> index to determine the highlighted text's occurrence.

We can get that occurrence value by getting the mouse click event coordinates. Then find the <span> located on that coordinates and retrieve its index.

We get the highlighted text with:

var text;
if(window.getSelection){
    text = window.getSelection().toString();
}else if (document.selection && document.selection.type != 'Control'){
    //for IE prior to 9
    text = document.selection.createRange().text;
}

Then check if the text is not empty or not spaces and wrap all similar text with span:

if($.trim(text).length){
    //wrap each word similar to the highlighted text with <span>
    var regex = new RegExp(text,'g')
    $('body').html($('body').html().replace(regex, '<span>'+text+'</span>'));
}

Now it's all wrapped, we need to get the original click coordinates:

$('body').on('click',function(e){
//get the x and y coords
var offset = $(this).offset(),
    x = e.clientX - offset.left,
    y = e.clientY - offset.top,
})

Get the new span located on that coordinates with:

var el = document.elementFromPoint(x, y)

Here is the jsfiddle