Why Are Props for Layout and Page Components Not Predefined in Next.js?

[PeterJSung]

I am currently using Next.js and have observed that there are no predefined Props for Layout and Page components, such as children and params. In my development experience, having these Props explicitly defined could enhance error checking and improve code clarity.

For example, a Layout component might be structured as follows:

export type NextLayoutProps = React.PropsWithChildren<{
    params:Promise<T>
}>


export default async function Layout({ children, params }: NextLayoutProps) {
  // Component logic here
}

I am curious to understand why these Props are not predefined in Next.js. Are there specific design considerations or technical constraints that have influenced this decision? Gaining insight into this would be greatly appreciated.

Additionally, I understand that the params in the Layout have recently been changed to a Promise type. If a predefined NextLayoutProps had existed, and params were updated to a Promise type, any resulting errors would have been immediately apparent to users during updates.

Thank you.

[icyJoseph]

There's some work on going here:

• feat: generate route types manifest #81396

I can only speculate that amongst every thing that needed to be done, although still important, this was not near the top of the stack, until recently.

Maybe it also carried some complexity from parallel routes, which can make several new entries to the layout and that any number of dynamic segments can also change the params seen by pages too.

Also, it is likely that a number of other technical bridges were implemented that now make this API possible.

[PeterJSung]

Thanks for the reply. I know that there may be problems due to parallel slug issues, but I think it can be solved using generics. i`ll waiting that pr