monaco-editor/website/playground/new-samples/interacting-with-the-editor/rendering-glyphs-in-the-margin/sample.js

var jsCode = [
	'"use strict";',
	'function Person(age) {',
	'	if (age) {',
	'		this.age = age;',
	'	}',
	'}',
	'Person.prototype.getAge = function () {',
	'	return this.age;',
	'};'
].join('\n');

var editor = monaco.editor.create(document.getElementById('container'), {
	value: jsCode,
	language: 'javascript',
	glyphMargin: true
});

var decorations = editor.deltaDecorations(
	[],
	[
		{
			range: new monaco.Range(3, 1, 3, 1),
			options: {
				isWholeLine: true,
				className: 'myContentClass',
				glyphMarginClassName: 'myGlyphMarginClass'
			}
		}
	]
);

// You can now use `decorations` to change or remove the decoration