One of the most used ways to check the “type” of a property which is passed to a React component is through prop-types . The main problem is that all the type checking is done on run-time, and not statically.

This article will explain how to go from type checking with prop-types to static type checking with Flow in React.

Why Flow?

The first and foremost reason for choosing Flow is that is opt-in. You can choose to type check a file or not.

When a project is already alive and well the last thing we want to do is rewrite everything. Flow enables you to add type checking step by step into your application.

This is also true for Typescript, but I choose Flow because it’s not a compiler, simply a checker. Babel can take care of the extra annotations.

Type checking is for some a missing part in JavaScript, which some developers might question. Is it really necessary? Does it make my code any better? Isn’t type checking just a tool for the developer? (Yes it is)

The main reason I choose to add a tool like Flow to my projects is to ensure I think about what goes in and goes out throughout my application. Type strictness helps my IDE understand what I type and what I should pass in function and props of React Components.

In a way I am already documenting my code, which makes my developer self quite happy if I look back at components I made a month ago.

Because Flow checks types statically it can be run outside of the run-time environment. This makes type checking errors in the browser a thing of the past and can be detected before building the application.

Type checking with prop-types

Consider the following setup, done with plain old prop-types .

import React from 'react' ; import { bool, node } from 'prop-types' ; const propTypes = { visible : bool, children : node.isRequired, }; const Toggle = ( { visible, children } ) => ( < div style = {{ display: visible ? ' block ' : ' none ' }}> { children } </ div > ); Toggle.propTypes = propTypes; const otherComponent = () => ( < section > < Toggle visible = true /> {/* Error at run-time! Missing children */} < Toggle visible = "true" > {/* Error at run-time! Not boolean */} ... </ Toggle > < Toggle visible > {/* Passes */} ... </ Toggle > </ section > );

In my opinion, throwing this error at run-time, most of the time in the browser, is kind of too late.

Reporting the mistake a developer has made in the IDE or different tool he’s using is a lot more convenient. This also helps with components other developers made and how to use them.

Type strictness here would also be documenting the use of the component.

Type checking with Flow

Let’s rewrite the previous example using Flow.

import React from 'react' ; import type { Element, Children } from 'react' ; type toggleType = { visible?: boolean, children : Children, }; const Toggle = ({ visible, children }: toggleType): Element<any> => ( < div style = {{ display: visible ? ' block ' : ' none ' }}> { children } </ div > ); const otherComponent = (): Element<any> => ( < section > < Toggle visible = true /> {/* Error! Missing children */} < Toggle visible = "true" > {/* Error! Not boolean */} ... </ Toggle > < Toggle visible > {/* Passes */} ... </ Toggle > </ section > );

All the errors in the last example would have been reported in the IDE or when you run Flow. And yes: Flow supports JSX, so that’s pretty awesome!

In closing

Flow makes it easier for me to enforce the correct use of my components in my applications. Adding type checking on other parts of your application is also a great idea, as you might grow more aware of the code you’re writing.

There is a great getting started guide on Flow’s website. Also a page on how to get Flow up and running in your favourite editor.

Happy checking!