promise

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