From @niieani on July 13, 2016 10:45

• VSCode Version: all versions, tested also on latest: 1.4.0-insider 50a0945
• OS Version: OS X 10.11

Steps to Reproduce:

1. Create a TypeScript file with a .js extension
2. Change the language mode inside VSCode to "TypeScript" (defaults to JavaScript in files with a .js extension)
3. Add some TypeScript specific code, e.g. interface ABC {}

Causes an error:

'interface declarations' can only be used in a .ts file.

Mentioned in the discussion here: #7926

Copied from original issue: microsoft/vscode#9199