Promise封装原生ajax

  原生ajax是XMLHttpRequest，监听readyStateChange事件，关注readyState和status。

const ajax = url => { return new Promise((resolve, reject) => { const xhr = new XMLHttpRequest() xhr.open('GET', url, true) // open xhr.setRequestHeader('Accept', 'application/json') xhr.onreadystatechange = () => { if (xhr.readyState !== 4) { return } if (xhr.status === 200 || xhr.status === 304) { resolve(xhr.responseText) } else { reject(new Error(xhr.responseText)) } } xhr.send() // send }) }