一种支持旧浏览器的解决方案:

function httpRequest() {
    var ajax = null,
        response = null,
        self = this;

    this.method = null;
    this.url = null;
    this.async = true;
    this.data = null;

    this.send = function() {
        ajax.open(this.method, this.url, this.asnyc);
        ajax.send(this.data);
    };

    if(window.XMLHttpRequest) {
        ajax = new XMLHttpRequest();
    }
    else if(window.ActiveXObject) {
        try {
            ajax = new ActiveXObject("Msxml2.XMLHTTP.6.0");
        }
        catch(e) {
            try {
                ajax = new ActiveXObject("Msxml2.XMLHTTP.3.0");
            }
            catch(error) {
                self.fail("not supported");
            }
        }
    }

    if(ajax == null) {
        return false;
    }

    ajax.onreadystatechange = function() {
        if(this.readyState == 4) {
            if(this.status == 200) {
                self.success(this.responseText);
            }
            else {
                self.fail(this.status + " - " + this.statusText);
            }
        }
    };
}

这段代码可能有点过分，但绝对是安全的。

用法:

//create request with its porperties
var request = new httpRequest();
request.method = "GET";
request.url = "https://example.com/api?parameter=value";

//create callback for success containing the response
request.success = function(response) {
    console.log(response);
};

//and a fail callback containing the error
request.fail = function(error) {
    console.log(error);
};

//and finally send it away
request.send();

为了刷新来自joann的最佳答案，这是我的代码:

let httpRequestAsync = (method, url) => {
    return new Promise(function (resolve, reject) {
        var xhr = new XMLHttpRequest();
        xhr.open(method, url);
        xhr.onload = function () {
            if (xhr.status == 200) {
                resolve(xhr.responseText);
            }
            else {
                reject(new Error(xhr.responseText));
            }
        };
        xhr.send();
    });
}

一个复制粘贴的现代版本(使用fetch和箭头函数):

//Option with catch
fetch( textURL )
   .then(async r=> console.log(await r.text()))
   .catch(e=>console.error('Boo...' + e));

//No fear...
(async () =>
    console.log(
            (await (await fetch( jsonURL )).json())
            )
)();

复制粘贴的经典版本:

let request = new XMLHttpRequest();
request.onreadystatechange = function () {
    if (this.readyState === 4) {
        if (this.status === 200) {
            document.body.className = 'ok';
            console.log(this.responseText);
        } else if (this.response == null && this.status === 0) {
            document.body.className = 'error offline';
            console.log("The computer appears to be offline.");
        } else {
            document.body.className = 'error';
        }
    }
};
request.open("GET", url, true);
request.send(null);

要做到这一点，建议使用Fetch API，使用JavaScript Promises。XMLHttpRequest (XHR)、IFrame对象或动态<script>标记是较旧(且较笨重)的方法。

<script type=“text/javascript”> 
    // Create request object 
    var request = new Request('https://example.com/api/...', 
         { method: 'POST', 
           body: {'name': 'Klaus'}, 
           headers: new Headers({ 'Content-Type': 'application/json' }) 
         });
    // Now use it! 

   fetch(request) 
   .then(resp => { 
         // handle response 
   }) 
   .catch(err => { 
         // handle errors 
    });
</script>

这里有一个很棒的获取演示和MDN文档

没有回调的版本

var i = document.createElement("img");
i.src = "/your/GET/url?params=here";

<button type="button" onclick="loadXMLDoc()"> GET CONTENT</button>

 <script>
        function loadXMLDoc() {
            var xmlhttp = new XMLHttpRequest();
            var url = "<Enter URL>";``
            xmlhttp.onload = function () {
                if (xmlhttp.readyState == 4 && xmlhttp.status == "200") {
                    document.getElementById("demo").innerHTML = this.responseText;
                }
            }
            xmlhttp.open("GET", url, true);
            xmlhttp.send();
        }
    </script>