Default Props in React/TypeScript

[DISCLAIMER: My dev experience is quite substantial, but I just started doing TypeScript, oh... about 3 weeks ago. So if I've screwed something up in this article, please feel free to call out my stooopidity in the comments.]

I just ran into something very... odd. It's one of those programming moments that makes you stop and say, "Wait... It can't really be like this, right???" It has to do with implementing default values for component props in React/TypeScript.

The Setup

Our team just began a brand new, "green fields" project. It will be written in React. (Great! That's my specialty.) Specifically, it will use TypeScript & React. (Umm... OK. I got some learnin' to do.) I've been wanting to get my feet wet in a TS project for awhile. So I've been diving in eagerly. But in the last week-or-so, something really threw me for a loop.

To illustrate the issue, I'm gonna take something from a plain-ol' JS component, and convert it into a TS component. The stub of my JS component looks like this:

export default function MyJSComponent(props) {
   return (
      <>
         Here is MyJSComponent:<br/>
         {props.children}
      </>
   );
}

MyComponent.propTypes = {
   requiredString: PropTypes.string.isRequired,
   requiredNumber: PropTypes.number.isRequired,
   optionalBoolean: PropTypes.bool,
   optionalString: PropTypes.string,
   optionalNumber: PropTypes.number,
};

MyComponent.defaultProps = {
   optionalBoolean: true,
   optionalString: 'yo',
   optionalNumber: 42,
};

Nothing fancy here. A dead-simple component that accepts up to 5 props, with 2 of those props being required. For the 3 optional props, default values are assigned. If the component is wrapping other content, that content will be rendered with props.children. This is basically React 101.

So let's set about converting this to TypeScript. In TS, we can infer data types right in the function signature. And in TS, just as in JS, we can supply default values for optional parameters - right in the function signature. So that could look something like this:

export default function MyTSComponent(
   requiredString: string,
   requiredNumber: number,
   optionalBoolean: boolean = true,
   optionalString: string = 'yo',
   optionalNumber: number = 42,
) {
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

Except... that doesn't work, does it? This fails on two key levels:

1. When React invokes a component, it doesn't supply the props to the components as an array of arguments. It supplies them in a single object - the props object. So TS will complain about the above code because it will realize that the props object does not correlate with the requiredString type of string.
   

2. The above code obliterates the standard React convention of being able to call props.children. We haven't defined any of the arguments as props, and therefore, there is no props.children to render.
   

In other words, the approach above works great when we're writing a "regular" TS function. But it won't work for a TS/React component. We'll need to account for the fact that all of the props are being passed into the component as a single object.

One approach is to alter your tsconfig.json to disable strict mode and allow implicit any types. That would look like this:

export default function MyTSComponent(props) {
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

With all of your configs disabled/relaxed, you can actually get the above code to run/compile. But if your solution to TS problems is to disable the power of TS, then... don't use TS.

And if your answer to coding problems in any language is to turn off strict mode or to relax the core config constructs... Well, let's just say that nothing in this article - or this entire site - is going to help you in any way.

Assuming that you're not in favor of disabling TS's core strengths, the next step is to figure out how to get TS to "accept" that props object. In other words, we need to explicitly define what's in props.

Inline Type-Hinting

I believe that, in TS, whenever possible, it's best if you can define your data types right in the function signature. It's efficient. It's easy for other devs to "grok". So now that we know that we must specifically define the props object being passed in, maybe we can do this?

export default function MyTSComponent(props: {
   requiredString: string, 
   requiredNumber: number, 
   optionalBoolean: boolean = true, 
   optionalString: string = 'yo',
   optionalNumber: number = 42,
   children: JSX.Element,
}) {
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

Except... that doesn't work, does it? If you try typing this out in your IDE, you'll notice that it does, for the most part, work - until you reach the point where you're trying to define default values on the optional properties. (Also, even if the default values worked, the idea of having to manually define props.children is just... yuck.)

Interfaces

It seems to me that interfaces are the "default" TypeScript way to handle these kinda situations. With a good interface, you can definitively type all the values that are expected in React's traditional props object. After tinkering with many different configurations, this is what I came up with:

interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

export default function MyTSComponent({
   requiredString,
   requiredNumber,
   optionalBoolean = true,
   optionalString = 'yo',
   optionalNumber = 42,
   children,
}: Props) {
   return (
      <>
         Here is MyComponent:<br/>
         {children}
      </>
   );
}

Unlike the other attempts shown above, this one actually works. React knows which values are required and which ones are optional. TypeScript understands the type associated with each argument. But IMHO, this approach still has... problems.

1. The full list of properties is spelled out twice - once in the interface, and once in the function signature. This is necessary because, if we neglect to list, say, requiredString in the interface, then TS won't know what type to assign to it. And if we neglect to list requiredString in the function signature, it simply won't be available anywhere within the function.
   

2. We have to list children in the function signature. For a long-time React guy, that just feels... wrong. It would be like having to define the console.log() method before you can use it. In React, children is supposed to be something that you just get "for free".
   

3. Speaking of React conventions, object destructuring obliterates the near-universal React practice of referencing props.foo or props.children. That may not be a big deal to some. But for me, it's huge. When I'm combing through the logic in a component, I definitely want to have a clear indicator that a particular variable was passed into the component as a prop. Once you destructure the props out of their original object, you lose that clear scoping.
   
   

defaultProps

You may be thinking, "If you want default prop values, why don't you just use the built-in functionality for defaultProps??" I certainly investigated this. It would look like this:

interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

const defaultProps: Props = {
   requiredString: '',
   requiredNumber: 0,
   optionalBoolean: true,
   optionalString: 'default',
   optionalNumber: 42,
}

const MyTSComponent: React.FC<Props> = (props) => {
   console.log(props);
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
};

MyTSComponent.defaultProps = defaultProps;

export default MyTSComponent;

There's a lot to like here. It maintains the traditional props convention. It doesn't require explicitly defining props.children. It makes for a clean function signature.

One thing I don't like about this approach is that I couldn't seem to get it to work unless I also defined default values inside defaultProps for the required props. If I remove requiredString and requiredNumber from the defaultProps definition, TS complains about it. Still - that's not really that big of a deal.

So is this the end of the article? The "real" solution for default props in React/TS? Umm... no.

No sooner did I start researching this pattern than I found out that there's a big push to deprecate defaultProps on functional components.

Given the issues I've outlined above, I don't honestly understand why anyone would want to deprecate defaultProps on functional components. They say things like, "default values are already handled in the function signature". Umm... no, they're not (at least not in a way that properly accommodates React's props object).

Regardless of the twisted reasoning behind this, it does seem as though this deprecation might happen. So with a big sigh, I moved on to search for other solutions.

My WTF?!?! Moment

Honestly, at this point, I started getting pretty annoyed. What I'm trying to do is, in React/JS, a five-minute lesson. When you first start doing React with plain-ol' JavaScript, it takes mere minutes to realize how you can set default values on the optional props. And yet, in React/TS, this seemingly-simple operation requires jumping through a ridiculous number of hoops. How can this be???

Imagine you travel to another country - one that speaks a language very similar to your own. While you're there, you say to your tour guide, "In your language, how do I say 'thank you'?" And the tour guide points you to a dozen different web pages that all explain ways that you can try to say 'thank you' - with no definitive answer. Finally, the tour guide says, "Well, in our variant of the language, there's really no simple way to say 'thank you'."

What???

It's not like I'm trying to migrate from JavaScript to Objective-C, or from JavaScript to C++. I'm merely moving from React/JS to React/TS. And I'm trying to do something that really should be drop-dead simple. And yet... I'm burning many many hours trying to solve this most-basic of questions.

Nevertheless, I pushed onward. The fact that this "problem" feels ridiculous to me doesn't do anything to help me solve the problem.

In-Function Processing

At this point, I started to think of "other" ways that I could provide default values. So I looked at applying them inside of the function itself. That looks like this:

interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

export default function MyTSComponent(props: Props) {
   props.optionalBoolean = props.optionalBoolean !== undefined ? props.optionalBoolean : true;
   props.optionalString = props.optionalString !== undefined ? props.optionalString : 'yo';
   props.optionalNumber = props.optionalNumber !== undefined ? props.optionalNumber : 42;
   console.log(props);
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

This doesn't throw any TS linting errors. However, it won't run, because React complains that the props object is not extensible. So, to get around that, we can do a deep clone of props with a cloneObject() function that I outlined in one of my previous articles.

[Yeah, yeah - I get it. Cloning props just so I can manually add default values feels a bit... hack-ish. But I'm just outlining a progression-of-thought here.]

So with an extra line to clone the props object, the code looks like this:

interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

export default function MyTSComponent(props: Props) {
   props = cloneObject(props);
   props.optionalBoolean = props.optionalBoolean !== undefined ? props.optionalBoolean : true;
   props.optionalString = props.optionalString !== undefined ? props.optionalString : 'yo';
   props.optionalNumber = props.optionalNumber !== undefined ? props.optionalNumber : 42;
   console.log(props);
   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

This approach... works. It compiles. It preserves the conventional props object, along with props.children. And for about a day-or-two, I really thought that this was the answer.

Then I started noticing a few annoyances...

While the above code indeed "works" just fine, I found that things started to get wonky when I began adding functions inside the functional component. Consider this example:

interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

export default function MyTSComponent(props: Props) {
   props = cloneObject(props);
   props.optionalBoolean = props.optionalBoolean !== undefined ? props.optionalBoolean : true;
   props.optionalString = props.optionalString !== undefined ? props.optionalString : 'yo';
   props.optionalNumber = props.optionalNumber !== undefined ? props.optionalNumber : 42;
   console.log(props);

   const getLetterArrayFromOptionalString = (): Array<string> => {
      return props.optionalString.split(''); 
   };

   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

I've set a default value of 'yo' on props.optionalString. And inside getLetterArrayFromOptionalString(), I'm trying to split() that string into an array of letters. But TS won't compile this. It complains that the props.optionalString object is possibly undefined - even though I clearly defined a default value at the top of the function.

Why does it do this?? Well, TS sees the function as being bound at the point when the component is mounted. And at the point that the component is mounted, there has been no default value set for props.optionalString yet. It doesn't matter that getLetterArrayFromOptionalString() will never be called until after a default value has been added to props.optionalString. TS doesn't fully grok that.

TS chokes on this because the split() function requires a type string | RexExp. But props.optionalString is type: string | undefined.

Where did that | undefined come from in our props.optionalString type? It was dynamically added by TS because the optionalString parameter is defined as optional (i.e., with the ? appended to it).

When you add ? to an interface property, TS will append | undefined as part of the type definition. This may seem like a good thing, but it can cause headaches later on because TS will expect you to write a whole bunch of code that's tolerant of undefined values - even though you know that you manually set a value for the variable, and it will never be undefined.

Annnnnd... I'm right back to the drawing board.

Finally - A Solution

For the time being, I think I have a working solution. (Until I find some other edge case where everything gets borked up...) It looks like this:

//all.props.requires.ts
export type AllPropsRequired<Object> = {
   [Property in keyof Object]-?: Object[Property];
};

// my.ts.component.tsx
interface Props extends PropsWithChildren<any>{
   requiredString: string,
   requiredNumber: number,
   optionalBoolean?: boolean,
   optionalString?: string,
   optionalNumber?: number,
}

export default function MyTSComponent(props: Props) {
   const args: AllPropsRequired<Props> = {
      ...props,
      optionalBoolean: props.optionalBoolean !== undefined ? props.optionalBoolean : true,
      optionalString: props.optionalString !== undefined ? props.optionalString : 'yo',
      optionalNumber: props.optionalNumber !== undefined ? props.optionalNumber : 42,
   };
   console.log(args);

   const getLetterArrayFromOptionalString = (): Array<string> => {
      return args.optionalString.split('');
   };

   return (
      <>
         Here is MyComponent:<br/>
         {props.children}
      </>
   );
}

So what's actually happening here?

The first thing you see is the AllPropsRequired type. In TS, it's something called a partial. I'm not gonna go through a tutorial on that here. Suffice it to say that AllPropsRequired is a type that will make all the properties of some other generic interface required. That will be important in a second...

The Props interface is fairly "standard" - nothing too magical there.

Inside MyTSComponent, the first thing I'm doing is creating a new object, based on the props, cast to the type AllPropsRequired. In other words, in the args object, I'm stripping out that optional ? indicator on every one of the properties.

I'm doing this because every property either had a value passed in (if it was required), or it will have a default value added to it. So none of the properties should be undefined, and we don't want the property's type to reflect that it could possibly be undefined.

Inside the args definition, the first thing I do is spread the ...props object. I do this so I can avoid manually spelling out each one of the required properties in the object. I only want to spell out the optional properties, and spreading ...props allows me to do that.

Then, for each of the optional properties, I'm checking to see if something was passed in. If nothing was passed in (i.e., if the property is undefined), I set the value to its default.

This approach preserves my props.children feature - because I've done nothing to alter/destroy the original props object. But throughout the component, any other time when I want to reference props, I'll use the args object.

This code compiles, and the line:

return args.optionalString.split('');

runs just fine. It doesn't throw any errors because, in the args object, optionalString doesn't have a type of string | undefined. It simply has a type of string.

It Shouldn't Be This Hard

Maybe I'm missing something here. Maybe, in the next week or two, I'll realize how silly this whole little journey was. Someone in the comments will say something like, "Why didn't you just use setDefaultProps()?" And I'll feel really silly for having burned several days trying to reinvent the wheel.

But I know that I'm not entirely alone in this. If you google around for things like "typescript default props functional components", you'll find a number of articles and Stack Overflow questions that (attempt to) address this same problem. And they all run into the same limitations. It just kinda feels like an... oversight to me.

And don't even get me started on the push to deprecate defaultProps for functional components. That just feels ridiculous to me. Or maybe it's not - I dunno. It could just be that something's not "clicking" right in my brain...

[NOTE: A few days after this was posted, I came up with an improved/revised method. That's highlighted in part two of this series...]