Insert text at cursor in a content editable div

UPD: since ~2020 solution is obsoleted (despite it can work yet)

// <div contenteditable id="myeditable">
// const editable = document.getElementById('myeditable')
// editable.focus()
// document.execCommand('insertHTML', false, '<b>B</b>anana')
document.execCommand('insertText', false, 'banana')

1. Get a Selection Object with window.getSelection().
2. Use Selection.getRangeAt(0).insertNode() to add a textnode.

3. If necessary, move the cursor position behind the added text with Selection.modify(). (Not standardized, but this feature is supported in Firefox, Chrome and Safari)

   function insertTextAtCursor(text)
   {
       let selection = window.getSelection();
       let range = selection.getRangeAt(0);
       range.deleteContents();
       let node = document.createTextNode(text);
       range.insertNode(node);
   
       for(let position = 0; position != text.length; position++)
       {
           selection.modify("move", "right", "character");
       };
   }
   

The following function will insert text at the caret position and delete the existing selection. It works in all the mainstream desktop browsers:

function insertTextAtCaret(text) {
    var sel, range;
    if (window.getSelection) {
        sel = window.getSelection();
        if (sel.getRangeAt && sel.rangeCount) {
            range = sel.getRangeAt(0);
            range.deleteContents();
            range.insertNode( document.createTextNode(text) );
        }
    } else if (document.selection && document.selection.createRange) {
        document.selection.createRange().text = text;
    }
}

UPDATE

Based on comment, here's some code for saving and restoring the selection. Before displaying your context menu, you should store the return value of saveSelection in a variable and then pass that variable into restoreSelection to restore the selection after hiding the context menu and before inserting text.

function saveSelection() {
    if (window.getSelection) {
        sel = window.getSelection();
        if (sel.getRangeAt && sel.rangeCount) {
            return sel.getRangeAt(0);
        }
    } else if (document.selection && document.selection.createRange) {
        return document.selection.createRange();
    }
    return null;
}

function restoreSelection(range) {
    if (range) {
        if (window.getSelection) {
            sel = window.getSelection();
            sel.removeAllRanges();
            sel.addRange(range);
        } else if (document.selection && range.select) {
            range.select();
        }
    }
}