Textbox in easyUI inserts content at the cursor position

<input id="fileName" name="filename" class="easyui-textbox lg_width" data-options="tipPosition:'bottom'" type="text" >

title = "${aaa}";// What to insert 
insertLabelText("fileName",title);// Calling method ,fileName by id="fileName" Of input label 
 
function insertLabelText(destobj,labelText) {
    
    let lastIndex = getCursortPosition(destobj);// Get cursor position 
    let oldText = $('#'+destobj).textbox('getValue');// Get the value of the input box 
    let newText = buildNewText(oldText, labelText, lastIndex);// Splice the value of the input box with the value to be inserted 
    $('#'+destobj).textbox('setValue',newText);// Set the spliced value to the input box 
}
// Get cursor position 
function getCursortPosition(destobj) {
    
    var cursorIndex = "-1";
    var obj = $('#'+destobj).next()[0].children[0]; //easyui Generated controls , All the information entered is on this 
    if (document.selection) {
    //IE browser 
        obj.focus ();
        var range= document.selection.createRange();
        range.moveStart ('character', -obj.value.length);
        cursorIndex= range.text.length;
    }else if (obj.selectionStart || obj.selectionStart==0) {
    // Not IE browser 
        cursorIndex= obj.selectionStart;
    }
    return cursorIndex;
}
// Concatenate new strings 
function buildNewText(oldText,labelText,lastIndex){
    
    let newText = oldText.substring(0, lastIndex) + labelText;// Insert at the beginning and end 
    if(oldText.length!=lastIndex&&oldText.length!=0){
    // Insert in the middle of a character 
        newText += oldText.substring(lastIndex, oldText.length);
    }
    return newText;
}