<form action="">
        <textarea name="textBox" id="" cols="30" rows="10"></textarea>
    </form>
    <script>
        var textBox = document.forms[0].elements['textBox'];
        function getSelectedText(textBox) {
            
            /*selectionStart selectionEnd 是HTML5新增方法*/
            if (typeof textBox.selectionStart === 'number') {
                return textBox.value.substring(textBox.selectionStart, textBox.selectionEnd);
            } 

            /*不支持HTML5的版本*/
            else if (window.getSelection) {
                return window.getSelection().toString();
            }
           /*兼容ie*/
            else if (document.selection) {
                return document.selection.createRange().text;
            }
        }
        textBox.addEventListener('select', function () {
            console.log(getSelectedText(this));
        }, false);

          /*选择指定的文本*/
        function SelectText(textBox, startIndex, endIndex) {
            if (textBox.setSelectionRange) {
                textBox.setSelectionRange(startIndex, endIndex);
            } 
            /*兼容ie，创建selection对象*/
            else if (textBox.createTextRange) {
                var range = textBox.createTextRange();
                range.collapse(true);
                range.moveStart('character', startIndex);
                range.moveEnd('character', endIndex - startIndex);
                range.select();
            }
            textBox.focus();
        }

https://www.cnblogs.com/ArthurPatten/p/3317263.html