"Feature can only be used in a .ts file." when switching the language mode to TypeScript in a .js file

[niieani]

• 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: microsoft/TypeScript#7926

[dbaeumer]

This is a problem with the TSServer that infers the language mode from the extension. There are discussion to make the customizable.

Moving to TS team.

[dbaeumer]

This issue was moved to microsoft/TypeScript#9670