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();
		}
	);
}