最简单的原生Ajax

var xhr = new XMLHttpRequest()
xhr.open('get', url, true)
xhr.send()
xhr.onreadystatuschange = function() {
    if (xhr.readyStatus === 4) {
        if(xhr.status === 200) {
            success(xhr.responseText)
        } else {
            fail && fail(xhr.status)
        }
    }
}

进阶 - promise方式的ajax

function ajax(url) {
    return new Promise(function(reslove, reject) {
        const xhr = new XMLHttpRequest()        xhr.open('GET', url)
        xhr.onload = function() {
            if (this.status === 200) {
                reselove(this.response)
            }
            reject(new Error(this.statusText))
        }
    })
}