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

[dbaeumer]

From @niieani on July 13, 2016 10:45

• VSCode Version: all versions, tested also on latest: 1.4.0-insider 50a0945d8de129472b154fa514ba5328897d3be1
• 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

[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.

[mhegazy]

There has been multiple requests around this area. I have files a new issue to consolidate all the requests along with a proposal for a fix, see #10939. Let's use #10939 to track this change.
closing in favor of #10939.