How do I load the contents of a text file into a javascript variable?

I have a text file in the root of my web app http://localhost/foo.txt and I'd like to load it into a variable in javascript.. in groovy I would do this:

def fileContents = 'http://localhost/foo.txt'.toURL().text;
println fileContents;

How can I get a similar result in javascript?

XMLHttpRequest, i.e. AJAX, without the XML.

The precise manner you do this is dependent on what JavaScript framework you're using, but if we disregard interoperability issues, your code will look something like:

var client = new XMLHttpRequest();
client.open('GET', '/foo.txt');
client.onreadystatechange = function() {
  alert(client.responseText);
}
client.send();

Normally speaking, though, XMLHttpRequest isn't available on all platforms, so some fudgery is done. Once again, your best bet is to use an AJAX framework like jQuery.

One extra consideration: this will only work as long as foo.txt is on the same domain. If it's on a different domain, same-origin security policies will prevent you from reading the result.

here is how I did it in jquery:

jQuery.get('http://localhost/foo.txt', function(data) {
    alert(data);
});

Update 2019: Using Fetch:

fetch('http://localhost/foo.txt')
  .then(response => response.text())
  .then((data) => {
    console.log(data)
  })

https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API

If you only want a constant string from the text file, you could include it as JavaScript:

// This becomes the content of your foo.txt file
let text = `
My test text goes here!
`;

<script src="foo.txt"></script>
<script>
  console.log(text);
</script>

The string loaded from the file becomes accessible to JavaScript after being loaded. The `(backtick) character begins and ends a template literal, allowing for both " and ' characters in your text block.

This approach works well when you're attempting to load a file locally, as Chrome will not allow AJAX on URLs with the file:// scheme.

Update 2020: Using Fetch with async/await

const response = await fetch('http://localhost/foo.txt');
const data = await response.text();
console.log(data);

Note that await can only be used in an async function. A longer example might be

async function loadFileAndPrintToConsole(url) {
  try {
    const response = await fetch(url);
    const data = await response.text();
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

loadFileAndPrintToConsole('https://threejsfundamentals.org/LICENSE');

This should work in almost all browsers:

var xhr=new XMLHttpRequest();
xhr.open("GET","https://12Me21.github.io/test.txt");
xhr.onload=function(){
	console.log(xhr.responseText);
}
xhr.send();

Additionally, there's the new Fetch API:

fetch("https://12Me21.github.io/test.txt")
.then( response => response.text() )
.then( text => console.log(text) )

One thing to keep in mind is that Javascript runs on the client, and not on the server. You can't really "load a file" from the server in Javascript. What happens is that Javascript sends a request to the server, and the server sends back the contents of the requested file. How does Javascript receive the contents? That's what the callback function is for. In Edward's case, that is

    client.onreadystatechange = function() {

and in danb's case, it is

 function(data) {

This function is called whenever the data happen to arrive. The jQuery version implicitly uses Ajax, it just makes the coding easier by encapsulating that code in the library.

When working with jQuery, instead of using jQuery.get, e.g.

jQuery.get("foo.txt", undefined, function(data) {
    alert(data);
}, "html").done(function() {
    alert("second success");
}).fail(function(jqXHR, textStatus) {
	alert(textStatus);
}).always(function() {
    alert("finished");
});

you could use .load which gives you a much more condensed form:

$("#myelement").load("foo.txt");

.load gives you also the option to load partial pages which can come in handy, see api.jquery.com/load/.