Flowtype Props Passed to Children in React

Here's how to type props for a child component that has props cloned onto it.

This is for you, @nguyenmanh1507! :)

React Clone Props onto Children

In React, you can send props to children. This is a way for a parent component to provide a child component with data it didn't already have.

Many times this is because a parent and a child component work in tandem to create some larger UI.

A Radio Button Example

An example of this is a radio button group that has many radio buttons. They are tied together by a name field, which allows the browser to know which radios belong to the group in order to cycle through them.

That code might look something like this:

import React from 'react'

const RadioOption = props =>
    <input type="radio" value={props.value} name={props.name} />

const RadioGroup = props =>
  <div class="radio-group">
    {React.Children.map(props.children, child =>
      child.type === RadioOption 
        ? React.cloneElement(child, {
            name: props.name
        : child

const WhereImUsingRadioGroups = _ => 
  <RadioGroup name="blizzard-games">
    <RadioOption label="Warcraft 2" value="wc2" />
    <RadioOption label="Warcraft 3" value="wc3" />
    <RadioOption label="Starcraft 1" value="sc1" />
    <RadioOption label="Starcraft 2" value="sc2" />

Flow Can't Infer the Cloned Prop

We can start type-checking this code with Flowtype by adding a comment to the top of the file:

// @flow

Flow immediately points out a problem to us:

                                                       ^ property `name`. Property not found in
                                                 ^ props of React element `RadioOption`

Flow can apparently infer that label and value are strings and passed in when RadioOption is used. But it is confused about name, which is cloned onto the RadioOption children inside RadioGroup.

Adding Types to RadioOption

We can provide some help to Flow by adding types to the props passed to RadioOption.

We can define them as:

type Props = {
  label: string,
  name?: string,
  value: string

And adjust the RadioOption signature to read:

const RadioOption = (props: Props) =>
  // ...

Now we have no Flow errors!

The name prop type is specified as name? to indicate it's an optional object property. This is required, or we reverse our problem, confusing Flow as to why the name is not included when

is called initially.

Are there other ways that you would handle typing cloned props in this case?