monaco-editor/website/playground/new-samples/extending-language-services/hover-provider-example/sample.js

monaco.languages.register({ id: 'mySpecialLanguage' });

monaco.languages.registerHoverProvider('mySpecialLanguage', {
	provideHover: function (model, position) {
		return xhr('../playground.html').then(function (res) {
			return {
				range: new monaco.Range(1, 1, model.getLineCount(), model.getLineMaxColumn(model.getLineCount())),
				contents: [
					{ value: '**SOURCE**' },
					{ value: '```html\n' + res.responseText.substring(0, 200) + '\n```' }
				]
			}
		});
	}
});

monaco.editor.create(document.getElementById("container"), {
	value: '\n\nHover over this text',
	language: 'mySpecialLanguage'
});

function xhr(url) {
	var req = null;
	return new Promise(function (c, e) {
		req = new XMLHttpRequest();
		req.onreadystatechange = function () {
			if (req._canceled) { return; }

			if (req.readyState === 4) {
				if ((req.status >= 200 && req.status < 300) || req.status === 1223) {
					c(req);
				} else {
					e(req);
				}
				req.onreadystatechange = function () { };
			}
		};

		req.open("GET", url, true);
		req.responseType = "";

		req.send(null);
	}, function () {
		req._canceled = true;
		req.abort();
	});
}