mouseDown，mouseMove，mouseUp更好的实践

不好的实践，move 不能太快，否则会监听不到

let flag = false
document.addEventListener('mousedown', down)
document.addEventListener('mousemove', move)
document.addEventListener('mouseup', up)

function down() {
  flag = true
  // ......
}
function move(e) {
  e.preventDefault()
  if(!flag) return
  // .....
}
function up() {
  flag = false
  // ......
}

较好的实践，这样可以快速的 mousemove，不会监听不到

document.addEventListener('mousedown', down)
document.addEventListener('mouseup', up)
function down() {
  document.addEventListener('mousemove', move)
  e.preventDefault()
}
function move(e) {
}
function up() {
  document.removeEventListener('mousemove', move)
}