es2015 version of insertAtCaret method

insertAtCaret.js

function insertAtCaret(text) {
    const txtarea = document.activeElement;
    const scrollPos = txtarea.scrollTop;
    let strPos = 0;

    const br = ((txtarea.selectionStart || txtarea.selectionStart == '0') ? "ff" : (document.selection ? "ie" : false ));

    if (br == "ie") { 
        txtarea.focus();
        const range = document.selection.createRange();

        range.moveStart('character', -txtarea.value.length);
        strPos = range.text.length;
    } else if (br == "ff") {
        strPos = txtarea.selectionStart;
    }

    const front = (txtarea.value).substring(0,strPos);  
    const back = (txtarea.value).substring(strPos,txtarea.value.length); 

    txtarea.value = front+text+back;
    strPos = strPos + text.length;

    if (br == "ie") { 
        txtarea.focus();

        const range = document.selection.createRange();
        range.moveStart ('character', -txtarea.value.length);
        range.moveStart ('character', strPos);
        range.moveEnd ('character', 0);
        range.select();
    } else if (br == "ff") {
        txtarea.selectionStart = strPos;
        txtarea.selectionEnd = strPos;
        txtarea.focus();
    }

    txtarea.scrollTop = scrollPos;
}