function getJson(url){
    return new Promise((resolve, reject) =>{
        var xhr = new XMLHttpRequest();
        xhr.open('open', url, true);
        xhr.onreadystatechange = function(){
            if(this.readyState == 4){
                if(this.status = 200){
                    resolve(this.responseText, this)
                }else{
                    var resJson = { code: this.status, response: this.response }
                    reject(resJson, this)    
                }
            }
        }
        xhr.send()
    })
}

function postJSON(url, data) {
    return new Promise( (resolve, reject) => {
        var xhr = new XMLHttpRequest()
        xhr.open("POST", url, true)
        xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

        xhr.onreadystatechange = function () {
            if (this.readyState === 4) {
                if (this.status === 200) {
                    resolve(JSON.parse(this.responseText), this)
                } else {
                    var resJson = { code: this.status, response: this.response }
                    reject(resJson, this)
                }
            }
        }

        xhr.send(JSON.stringify(data))
    })
}

getJSON('/api/v1/xxx')    
.catch( error => {
  // dosomething
})
.then( response => {
  // dosomething
})