Type Checking in JS File allows invalid JavaScript

monaco-editor version: Latest
Browser: Chrome
OS: Windows
Playground code that reproduces the issue: Extending Language Services > Configuring JavaScript Defaults

(Set noSemanticValidation to false on line 8)

// 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: false,
	noSyntaxValidation: false
});

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

// extra libraries
var libSource = [
	'declare class Facts {',
	'    /**',
	'     * Returns the next fact',
	'     */',
	'    static next():string',
	'}',
].join('\n');
var libUri = 'ts:filename/facts.d.ts';
monaco.languages.typescript.javascriptDefaults.addExtraLib(libSource, libUri);
// When resolving definitions and references, the editor will try to use created models.
// Creating a model for the library allows "peek definition/references" commands to work with the library.
monaco.editor.createModel(libSource, 'typescript', monaco.Uri.parse(libUri));

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

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

In this editor, the language is set to javascript and semantic validation is turned on for type checking. (Equivalent to // @ts-check in VSCode).

Since the compiler is TypeScript, it allows any valid TypeScript, which can be invalid JavaScript.

Take the following code block for example:

let x = 10;
x.nonExistantMethod();

let y: any = 20;

type MyType = string;

Expected Behaviour (from VSCode)

let x = 10;
x.nonExistantMethod(); // Error: Property 'nonExistantMethod' does not exist on type 'number'.

let y: any = 20; // Error: Type annotations can only be used in TypeScript files.

type MyType = string; // Error: Type aliases can only be used in TypeScript files.

Actual Behaviour (in playground)

let x = 10;
x.nonExistantMethod(); // Error: Property 'nonExistantMethod' does not exist on type 'number'.

let y: any = 20; // No Error

type MyType = string; // No Error