Vanilla JavaScript is untyped by nature, some will call it “smart” because it is able to figure out what is a number or a string.

This makes it easier to run JavaScript code, it simply needs to be executed in a browser or Node.js runtime. However, its vulnerable to numerous runtime errors that can spoil your user experience.

If you’ve ever experienced the following, you would benefit from adopting a type system.

• After fetching a list of data you may find that a certain field doesn’t exist on one of the records, causing the app to crash unless explicitly handled.
• An instance of a class you imported doesn’t have a method you’re trying to invoke.
• Your IDE doesn’t know what methods and properties are available, so it cannot easily assist you with autocomplete.
• Difficulty reasoning about code, type systems at a glance make it easier to refactor

Flow, TypeScript, or ReasonML

If you have an existing codebase that you wish to make bulletproof, with respect to type errors, you could try adopting Flow or TypeScript. They have a fairly similar syntax.