字符串字面量类型

字符串字面量类型允许你指定字符串必须的固定值。

type EventNames = 'click' | 'scroll' | 'mousemove'

function handler (eventType: 'click'): void
function handler (eventType: 'scroll'): void
function handler (eventType: 'mousemove'): void
function handler (eventType: EventNames): void {
  if (eventType === 'click') {
    // ...
  }
  else if (eventType === 'scroll') {
    // ...
  }
  else {
    // ...
  }
}

handler('click') // Ok
handler('dbclick') // Error