SOURCE

function ajax(url, method = 'GET') {
    const p = new Promise((resolve, reject) => {
        const xhr = new XMLHttpRequest()
        xhr.open(method, url, true)
        xhr.onreadystatechange = function () {
            if (xhr.readyState === 4) {
                if (xhr.status === 200) {
                    resolve(JSON.parse(xhr.responseText))
                } else if (xhr.status === 404 || xhr.status === 500) {
                    reject(new Error('404 not found'))
                }
            }
        }
        xhr.send(null)
    })
    return p
}

const url = '/data/test.json'
const url1 = 'https://www.baidu.com'
ajax(url)
    .then(res => console.log(res))
    .catch(err => console.log(err))


// function ajax(url){
//     const p = new Promise((resolve, reject) => {
//         const xhr = new XMLHttpRequest()
//         xhr.open('GET', url)
//     })
// }