monaco-editor/website/playground/new-samples/extending-language-services/configure-javascript-defaults/sample.js

// Add additonal d.ts files to the JavaScript language service and change.
// Also change the default compilation options.
// The sample below shows how a class Facts is declared and introduced
// to the system and how the compiler is told to use ES6 (target=2).

// validation settings
monaco.languages.typescript.javascriptDefaults.setDiagnosticsOptions({
	noSemanticValidation: true,
	noSyntaxValidation: false
});

// compiler options
monaco.languages.typescript.javascriptDefaults.setCompilerOptions({
	target: monaco.languages.typescript.ScriptTarget.ES6,
	allowNonTsExtensions: true
});

// extra libraries
monaco.languages.typescript.javascriptDefaults.addExtraLib([
	'declare class Facts {',
	'    /**',
	'     * Returns the next fact',
	'     */',
	'    static next():string',
	'}',
].join('\n'), 'filename/facts.d.ts');

var jsCode = [
	'"use strict";',
	'',
	"class Chuck {",
	"    greet() {",
	"        return Facts.next();",
	"    }",
	"}"
].join('\n');

monaco.editor.create(document.getElementById("container"), {
	value: jsCode,
	language: "javascript"
});