With VSCode 1.12 (aka the current insiders build) you can also enable JavaScript type checking without creating a jsconfig.json or even changing your source code by setting "javascript.implicitProjectConfig.checkJs":true in your workspace or user settings. This setting applies to any JavaScript file that is not part of a jsconfig or tsconfig project. Besides catching errors, checkJs also enables a number of quick fixes in JavaScript files.

Please let us know if you see any errors while trying out new TypeScript 2.3 features in VSCode or have any thoughts on how our JS/TS support could be improved: https://github.com/Microsoft/vscode/issues/new