[javascript] How to get the response of XMLHttpRequest?

I'd like to know how to use XMLHttpRequest to load the content of a remote URL and have the HTML of the accessed site stored in a JS variable.

Say, if I wanted to load and alert() the HTML of http://foo.com/bar.php, how would I do that?

I'd suggest looking into fetch. It is the ES5 equivalent and uses Promises. It is much more readable and easily customizable.

const url = "https://stackoverflow.com";
fetch(url)
    .then(
        response => response.text() // .json(), etc.
        // same as function(response) {return response.text();}
    ).then(
        html => console.log(html)
    );

In Node.js, you'll need to import fetch using:

const fetch = require("node-fetch");

If you want to use it synchronously (doesn't work in top scope):

const json = await fetch(url)
  .then(response => response.json())
  .catch((e) => {});

More Info:

Mozilla Documentation

Can I Use (95% Feb 2020)

Matt Walsh Tutorial

The simple way to use XMLHttpRequest with pure JavaScript. You can set custom header but it's optional used based on requirement.

1. Using POST Method:

window.onload = function(){
    var request = new XMLHttpRequest();
    var params = "UID=CORS&name=CORS";

    request.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            console.log(this.responseText);
        }
    };

    request.open('POST', 'https://www.example.com/api/createUser', true);
    request.setRequestHeader('api-key', 'your-api-key');
    request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    request.send(params);
}

You can send params using POST method.

2. Using GET Method:

Please run below example and will get an JSON response.

window.onload = function(){_x000D_
    var request = new XMLHttpRequest();_x000D_
_x000D_
    request.onreadystatechange = function() {_x000D_
        if (this.readyState == 4 && this.status == 200) {_x000D_
            console.log(this.responseText);_x000D_
        }_x000D_
    };_x000D_
_x000D_
    request.open('GET', 'https://jsonplaceholder.typicode.com/users/1');_x000D_
    request.send();_x000D_
}

In XMLHttpRequest, using XMLHttpRequest.responseText may raise the exception like below

 Failed to read the \'responseText\' property from \'XMLHttpRequest\': 
 The value is only accessible if the object\'s \'responseType\' is \'\' 
 or \'text\' (was \'arraybuffer\')

Best way to access the response from XHR as follows

function readBody(xhr) {
    var data;
    if (!xhr.responseType || xhr.responseType === "text") {
        data = xhr.responseText;
    } else if (xhr.responseType === "document") {
        data = xhr.responseXML;
    } else {
        data = xhr.response;
    }
    return data;
}

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
        console.log(readBody(xhr));
    }
}
xhr.open('GET', 'http://www.google.com', true);
xhr.send(null);