Examples of VSCode 1.12.1 TypeScript Types inferred from ES6 code

typed.es6

{
  /*
   * Examples of VSCode 1.12.1 TypeScript Types inferred from ES6 code.
   * Conclusion: You are not required to use JSDoc or TypeScript
   * annotations in most cases if you use default values in functions.
   */

  let easy = 'abc';

  // Error: Type '123' is not assignable to type 'string'
  easy = 123;

  // Can types be infered from ES6 default values rather than use JSDoc?
  const arrayId = (a = []) => a;

  arrayId([]);

  // Error: Argument of type 'string' is not assignable to parameter
  // of type 'any[]'
  arrayId(easy);

  // So long as the Class has an instance property it is correctly infered
  // as a Class type for checking in Function arguments.
  class Example {
    static is() {
      return 'Example';
    }
    get me() {
      return Example.is;
    }
  }

  const ex = new Example();
  const fakeEx = {
    set me(v) {
      this._me = v;
    },
    get me() {
      return this._me || 'fakeMe';
    },
  };

  /**
   * @param {Example} x
   * @return {Example}
   */
  const exampleId = (x) => x;

  // Error: Supplied parameters do not match any signature of call target.
  exampleId();
  // Error: Argument of type '""' is not assignable to parameter
  // of type 'Example'
  exampleId('');
  // Error: Argument of type 'typeof Example' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type 'typeof Example'.
  exampleId(Example);
  // Error: Argument of type '{}' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type '{}'.
  exampleId({});
  // Error: Argument of type '() => void' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type '() => void'.
  exampleId(() => {});
  // correct
  exampleId(ex);
  // correct
  exampleId(new Example());
  // correct Interface
  exampleId(fakeEx);

  const exampleIdClassDefault = (x = Example) => x;

  // correct - uses default value
  exampleIdClassDefault();
  // Error: Argument of type '""' is not assignable to parameter
  // of type 'Example'
  exampleIdClassDefault('');
  // Error: Argument of type '{}' is not assignable to parameter
  // of type 'typeof Example'.
  // Property 'prototype' is missing in type '{}'.
  exampleIdClassDefault({});
  // Error: Argument of type '() => void' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type '() => void'.
  exampleIdClassDefault(() => {});
  // correct
  exampleIdClassDefault(Example);
  // Error: Argument of type 'Example' is not assignable to parameter
  // of type 'typeof Example'.
  // Property 'prototype' is missing in type 'Example'.
  exampleIdClassDefault(ex);
  // Error: Argument of type 'Example' is not assignable to parameter
  // of type 'typeof Example'.
  exampleIdClassDefault(new Example());
  // Error: Argument of type '{ [x: string]: any; me: any; }' is not
  // assignable to parameter of type 'typeof Example'.
  // Property 'prototype' is missing in type '{ [x: string]: any; me: any; }'.
  exampleIdClassDefault(fakeEx);

  const exampleIdInstanceDefault = (x = new Example()) => x;

  // correct - uses default value
  exampleIdInstanceDefault();
  // Error: Argument of type '""' is not assignable to parameter
  // of type 'Example'
  exampleIdInstanceDefault('');
  // Error: Argument of type '{}' is not assignable to parameter
  // of type 'typeof Example'.
  // Property 'prototype' is missing in type '{}'.
  exampleIdInstanceDefault({});
  // Error: Argument of type '() => void' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type '() => void'.
  exampleIdInstanceDefault(() => {});
  // Error: Argument of type 'typeof Example' is not assignable to parameter of type 'Example'.
  exampleIdInstanceDefault(Example);
  // correct
  exampleIdInstanceDefault(ex);
  // correct
  exampleIdInstanceDefault(new Example());
  // correct Interface
  exampleIdInstanceDefault(fakeEx);

  const exampleIdInstanceDefaultVar = (x = ex) => x;

  // correct - uses default value
  exampleIdInstanceDefaultVar();
  // Error: Argument of type '""' is not assignable to parameter
  // of type 'Example'
  exampleIdInstanceDefaultVar('');
  // Error: Argument of type '{}' is not assignable to parameter
  // of type 'typeof Example'.
  // Property 'prototype' is missing in type '{}'.
  exampleIdInstanceDefaultVar({});
  // Error: Argument of type '() => void' is not assignable to parameter
  // of type 'Example'.
  // Property 'me' is missing in type '() => void'.
  exampleIdInstanceDefaultVar(() => {});
  // Error: Argument of type 'typeof Example' is not assignable to parameter of type 'Example'.
  exampleIdInstanceDefaultVar(Example);
  // correct
  exampleIdInstanceDefaultVar(ex);
  // correct
  exampleIdInstanceDefaultVar(new Example());
  // correct Interface
  exampleIdInstanceDefaultVar(fakeEx);
}