Getting and setting of a text selection in a textarea - ms-office

Question

I want to create a textarea where users can select a part of a text, and I will react according to their selection. So I need to
1) get the start and end positions of the selection text
2) get the position of the focus, if it is in the textarea and there is no selection
It seems that the functions to do so are different from an explorer to another. So could anyone tell me what is the approach to do that in Office Add-in?
I have tried the following 2 ways (ie, select a part of the text in myTextarea, click on button, and then debug the code), they don't seem to be the right functions.
(function() {
"use strict";
Office.initialize = function(reason) {
$(document).ready(function() {
app.initialize();
$('#button').click(showSelection);
});
};
function showSelection() {
// way 1
console.log(document.selection); // undefined
document.getElementById("myTextarea").focus();
var sel = document.selection.createRange(); // Uncaught TypeError: Cannot read property 'createRange' of undefined
selectedText = sel.text;
// way 2
console.log(document.getElementById("myTextarea").selectionstart); // undefined
console.log(document.getElementById("myTextarea").selectionend); // undefined
}
})();
Additionally, it would be great if one could also tell me how to realise the follows by code:
1) select a part of a text, from a start and end positions
2) set the focus at a certain position of the textarea
Edit 1:
I just tried window.getSelection() within my Excel add-in:
function showselection() {
var a = window.getSelection();
var b = window.getSelection().toString();
var c = window.getSelection().getRangeAt(0);
}
After selecting a text in the textarea, and clicking on button, I debugged step by step: the first line made a a = Selection {anchorNode: null, anchorOffset: 0, focusNode: null, focusOffset: 0, is ...; the second line returned "", the third line got an error Home.js:19 Uncaught IndexSizeError: Failed to execute 'getRangeAt' on 'Selection': 0 is not a valid index. It looks like the selection has not been successfully caught...
Here is JSBin without Excel add-in frame, which returns almost same results as above.

Answer 1

Use JQuery.
For instance, the following two lines get the caret position:
function showselection() {
console.log($('#myTextarea')[0].selectionStart);
console.log($('#myTextarea')[0].selectionEnd);
}
There are some plug-ins:
https://github.com/localhost/jquery-fieldselection
http://madapaja.github.io/jquery.selection/
The second one has several short samples with buttons (where we may lose selection). You could either use their API, or look into their code to see which JQuery functions they call.

Answer 2

If the desired selection is just the selected text in the HTML page (and not the user's selection in Excel/Word), then there are some good stackoverflow answers about accessing that selection.

Answer 3

One of the key features of the JavaScript APIs for Office is that they follow an asynchronous model (the code you've written above for showSelection() appears to be synchronous). I'd recommend reading the Excel and Word JS API overview pages to get a feel for how they work. As an example, here's how you'd get the text from a selection:
Word.run(function (context) {
var myRange = context.document.getSelection();
context.load(myRange, 'text');
return context.sync().then(function () {
log("Selection contents: " + myRange.text);
});
})
Then for the other specifics of your question please clarify as requested in my comment. Thanks!
-Michael (PM for Office add-ins)