The context

We have a Menu component that can have as children MenuItem or a custom react component. Let's say the signature of MenuItem is like this :

// MenuItem.tsx
export type MenuItemProps = {
... // some props
    label: string
}

export const MenuItem: React.FC<MenuItemProps> = ({ label }) => {
  return (
    <div>
      <h1>{label}</h1>
    </div>
  )
}

And the Menu component takes a list of MenuItem props and displays each MenuItem with its props:

// Menu.tsx
export type MenuProps = {
    ... // some props
    menuItemList: MenuItemProps[]
}

export const Menu: React.FC<MenuProps> = ({menuItemList}) => {
    return (
        <div>
            <h1>Menu</h1>
                <div>
                    {
                        menuItemList.map((menuItem, index) => <MenuItem key={index} {...menuItem} />)
                    }
                </div>
        </div>
    )
}

The problem

Now, I want that the user can display a custom component that he/she builds. Let's say a div with icon. We need children as props to Menu like this:

export type MenuProps = {
    ... // some props
    menuItemList: MenuItemProps[]
    children: React.ReactNode
}

But the problem: where do I put the children prop?

• Before or after the list of MenuItem ?
• Between the list of MenuItem ? (More pains for nothing)

Yes, because there will be some devs who will provide these two props even if there are optionals.

I finally decide: Only one of them will be available to use so "If user/dev set menuItemList array he/she cannot set children prop".

The solution: TypeScript + never + union

We can try to define these two optional props and then check their existence and raise an error if both are available. I did not like this and did not try to implement it. The solution I used has the same logic but cleaner.

If like me, you are wondering where we can use never, be patient, here you'll use it 🤣

The final type will be in two parts.

1st step

export type MenusProps = {
  children?: null | undefined,
  menuItemList?: MenuItemProps[],
}

We make all of them optionals. Since we want to base our condition to the availability of children, we make it optional as well.

2nd step

export type MenusProps = {
  children: React.ReactNode,
  menuItemList?: never,
}

With this type, MenuProps can have children as prop but not menuItemList.

3rd step

export type MenusProps =
  | {
      children?: null | undefined,
      menuItemList?: MenuItemProps[],
    }
  | {
      children: React.ReactNode,
      menuItemList?: never,
    }

We now indicate that MenuProps is an union of the above types. So it can have one of them but not both. In this case, if the user of Menu component provides children as prop, he/she cannot provides again menuItemList as prop and if he/she can pass nothing.

Bonus

The final type is

export type MenusProps =
  | {
      children?: null | undefined,
      menuItemList?: MenuItemProps[],
    }
  | {
      children: Exclude<React.ReactNode, null | undefined>,
      menuItemList?: never,
    }

A React.ReactNode can be null | undefined and so we have two optionals properties in the second union which is not the desired behavior. So we Exclude some values for children in this case and so we can write

// Menu.tsx
export type MenusProps =
  | {
      children?: null | undefined;
      menuItemList?: MenuItemProps[];
    }
  | {
      children: Exclude<React.ReactNode, null | undefined>;
      menuItemList?: never;
}


export const Menu: React.FC<MenuProps> = ({ menuItemList }) => {
  return (
    <div>
      <h1>Menu</h1>
      <div>

                {menuItemList && {menuItemList.map((menuItem, index) => (
          <MenuItem key={index} {...menuItem} />
        ))}}

                {children}
      </div>
    </div>
  )
}

Now, if you try to give a children and a menuItemList to Menu, TypeScript will display a type incompatibility warning.

Conclusion

This solution may not be the best but it helps us to make some props depends to another and TypeScript can raise a warning if user didn't respect this rule.

I share a few tips that I use in my daily tasks, and I hope you have others that you want to share with us. I'm open to tips, and my social accounts are there for that.

So can you smell what Tawaliou is cooking ?