Stuff I wish I knew when starting with Flow

Typing in dynamic languages is hard. Especially in small projects where interaction with libraries is hiding on every corner. I tried to get into Flow at least two times before. Now I finally think I got far enough to experience enough of important obstacles that suffice to write about them.

What editor should I use

Technically, both Atom and VS Code should work with Flow. However, VS Code wasn't working well for me. On the other hand Flow for Atom IDE works great.

Where to start

With Flow you can pick the file you want to start with. Flow will treat imported untyped files as type any. I started with a React component which accepts simple object just to get my feet wet and get familiar with the syntax. After this exploration phase it's good to try getting business logic typed.

Interacting with HTML elements

Finding the correct way to handle type casting of HTML elements took me surprisingly a lot of the time. But the solution turned out to be pretty simple. Flow looks for instanceof conditions and for following code it works with correct type. Useful tool for asserting HTML elements is invariant. It takes condition and error message and raises the error message when the condition isn't met.

const form = document.getElementById(formId)
  form != null && form instanceof HTMLFormElement,
  'Could not find a form.'

Arrays - type syntax can actually differ

When specifying a type for an array I automatically typed [Item]. This is valid syntax in Flow, but it means something which you won't need most of the time. It means array with one item of type Item. You are most likely looking for Item[]. This took me quite some time to realize.

Higher-Order Components

HOCs are sometimes tough to type correctly because it's so easy to mix in any type and lose the type checking. Let's try typing this withLoader HOC. I confess that I haven't figured out how to write fully generic HOC.

function withLoader(LoaderComponent) {
  function innerComponent(WrappedComponent) {
    return props =>
      props.loading === true ? (
        <loadercomponent {...props}=""></loadercomponent>
      ) : (
        <wrappedcomponent {...props}=""></wrappedcomponent>
  return innerComponent

It's just a condition wrapped in two functions, which shouldn't be that hard. First call supplies loader component and second call supplies the component we are wrapping.

const WrappedComponentWithLoader = withLoader(LoaderComponent)(WrappedComponent)

Looking at Flow docs I found that React uses two types to define component types. I will use React.StatelessFunctionalComponent to keep things "simple".

function withLoader(LoaderComponent: React.StatelessFunctionalComponent<any>) {
  function innerComponent<props: boolean="" loading:="" {="" }="">(
    WrappedComponent: React.StatelessFunctionalComponent<props>
  ): React.StatelessFunctionalComponent<props> {
    return props =>
      props.loading === true ? (
      ) : (
        <wrappedcomponent {...props}=""></wrappedcomponent>
  return innerComponent;

After adding types this HOC looks pretty intimidating. I will try to walk you through without scaring you off.

LoaderComponent: React.StatelessFunctionalComponent<any> means that function withLoader accepts any stateless component. Then there is my generic wrapped component. innerComponent<props: boolean="" loading:="" {="" }=""> part defines that function has generic type which is required to be object with loading field of type boolean. I used this type to define Props of my wrapped component. This is the only solution I found that passes up the types defined in decorated component so the type restrictions are correctly propagated and property loading is also required. The last line that is related to flow is ): React.StatelessFunctionalComponent<props> { which actually defines that new component will have type discussed previously.

You can play with this component on Try Flow and see that Flow warns us about missing props.