main.js

// @flow

const a: number = 'asdf';
const b: string = 2;

output

> flow version
Flow, a static type checker for JavaScript, version 0.24.0
> flow check
main.js:3
  3: const a: number = 'asdf';
                       ^^^^^^ string. This type is incompatible with
  3: const a: number = 'asdf';
              ^^^^^^ number

main.js:4
  4: const b: string = 2;
                       ^ number. This type is incompatible with
  4: const b: string = 2;
              ^^^^^^ string


Found 2 errors