光标处插入文本

function insert() {
  let element = document.getElementById('text')
      let value = '_'
      // IE support
      if (document.selection) {
        console.log('ie')
        element.focus()
        let sel = document.selection.createRange()
        sel.text = value
      } else if (element.selectionStart || element.selectionStart === '0') { // MOZILLA and others
        console.log('modern')
        let startPos = element.selectionStart
        let endPos = element.selectionEnd
        element.value = element.value.substring(0, startPos) + value + element.value.substring(endPos, element.value.length)
        element.selectionStart = startPos + value.length
        element.selectionEnd = startPos + value.length
      } else {
        element.value += value
      }
}