promise实现ajax操作

const getJson = function(url){
    const promise = new Promise((resolve,reject)=>{
        const handler = function(){
            if(this.readyState!==4){
                return;
            }
            if(this.status === 200){
                resolve(this.response);
            }else{
                reject(new Error(this.statusText));
            }
        }
        const client =  new XMLHttpRequest()
        client.open('get',url)
        client.onreadystatechange = handler
        client.responseType = 'json'
        client.setRequestHeader('Accept','application/json')
        client.send()
    })
    return promise
}
getJSON("/posts.json").then((json)=> {
  console.log('Contents: ' + json);
}.catch((err)=>{
  console.log('err: ' + err);
});