ajax封装成promise

let btn = document.getElementById("btn");
			let demo = document.getElementById("#demo");

			const ajax_post_promise = (url, parameter = {}) => {
				return new Promise(function(resolve, reject) {
					let xhr = new XMLHttpRequest();
					xhr.onreadystatechange = () {
						if (xhr.readyState == 4) {
							if ((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
								resolve(JSON.parse(xhr.responseText));
							}
						}
					}
					xhr.open('post', url, true);
					xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded')
					xhr.send(parameter.body);
				})
			};

			const ajax_get_promise = (url, parameter = {}) => {
				return new Promise((resolve, reject) => {
					let xhr = new XMLHttpRequest();
					let param = Object.entries(parameter).map((arrItem, index) => `?${arrItem[0]}=${arrItem[1]}`);
					
					xhr.onreadystatechange = () {
						if (xhr.readyState == 4) {
							if ((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
								resolve(JSON.parse(xhr.responseText));
							}
						}
					}
					xhr.open('get', url + param, true);
					xhr.send(null);
				})
			};


			ajax_post_promise('http://localhost:3020/users/')
				.then((res) => console.log(res))
				.catch((err) => console.log(err));

			ajax_get_promise(('http://localhost:3020/users/'))
				.then((res) => console.log(res))
				.catch((err) => console.log(err));