[javascript] js 'types' can only be used in a .ts file - Visual Studio Code using @ts-check

I am starting to use TypeScript in a Node project I am working on in Visual Studio Code. I wanted to follow the "opt-in" strategy, similar to Flow. Therefore I put // @ts-check at the top of my .js file in hope to enable TS for that file. Ultimately I want the same experience of "linting" as Flow, therefore I installed the plugin TSLint so I could see Intellisense warnings/errors.

But with my file looking like:

// @ts-check

module.exports = {
  someMethod: (param: string): string => {
    return param;
  },
};

and my tsconfig.json file looking like...

{
  "compilerOptions": {
      "target": "es2016",
      "module": "commonjs",
      "allowJs": true
  }
}

I get this error: [js] 'types' can only be used in a .ts file. as shown below in the image.

I saw this question which recommended disabling javascript validation in vscode but then that doesn't show me any TypeScript Intellisense info.

I tried setting tslint.jsEnable to true in my vscode settings as mentioned in the TSLint extension docs but no luck there.

What is the correct setup in order to use .js files with TypeScript and get Intellisense so I know what the errors in my code are before I run any TS commands?

You must use a .ts file - e.g. test.ts to get Typescript validation, intellisense typing of vars, return types, as well as "typed" error checking (e.g. passing a string to a method that expects an number param will error out).

It will be transpiled into (standard) .js via tsc.

Update (11/2018):

Clarification needed based on down-votes, very helpful comments and other answers.

types

• Yes, you can do type checking in VS Code in .js files with @ts-check - as shown in the animation

• What I originally was referring to for Typescript types is something like this in .ts which isn't quite the same thing:

  hello-world.ts

  function hello(str: string): string {
    return 1;
  }
  
  function foo(str:string):void{
     console.log(str);
  }
  

  This will not compile. Error: Type "1" is not assignable to String

• if you tried this syntax in a Javascript hello-world.js file:

  //@ts-check
  
  function hello(str: string): string {
    return 1;
  }
  
  function foo(str:string):void{
     console.log(str);
  }
  

  The error message referenced by OP is shown: [js] 'types' can only be used in a .ts file

If there's something I missed that covers this as well as the OP's context, please add. Let's all learn.

Use "javascript.validate.enable": false in your VS Code settings, It doesn't disable ESLINT. I use both ESLINT & Flow. Simply follow the instructions Flow For Vs Code Setup

Adding this line in settings.json. Helps "javascript.validate.enable": false

For anyone who lands here and all the other solutions did not work give this a try. I am using typescript + react and my problem was that I was associating the files in vscode as javascriptreact not typescriptreact so check your settings for the following entries.

   "files.associations": {
    "*.tsx": "typescriptreact",
    "*.ts": "typescriptreact"
  },

Just default the variable to the expected type:

(number=1) => ...
(number=1.0) => ...
(string='str') ...