定时器实现往复运动 模拟效果

start 到end 的往复运动（从current 开始，direct 为方向）

var start = 1var end = 9var current = 5var direct = truesetInterval(()=>{if (current < end && direct) {current += 1direct = true} else if (current >= end && direct) {direct = falsecurrent -= 1} else if (current > start && !direct) {current -= 1direct = false} else if (current <= start && !direct) {current += 1direct = true} console.log(current, '----')},1000)

start 到end 的往复运动，在start1和end1处会进行折返（从current 开始，direct 为方向）