字符串字面量类型来约束取值只能是某几个字符串中的一个。举个例子：
type EventName = 'click' | 'scroll' | 'mousemove';
function handleEvent(ele: Element, event: EventNames){
}
handleEvent(document.getElementById('hello'), 'scroll') //不会报错
handleEvent(document.getElementById('world'), 'dblclick') //报错，event不能为dblclick
