Imagine that we're working with an external library that has a new global element available to us. We want to strongly type the element so that it can only be used with the correct attributes.

In this case, we want to create a `<something>` element and give it a required `id` attribute, which is a string:

```tsx
<something id="123"></something>
```

However, when we use this element TypeScript tells us that it doesn't exist on the `JSX.IntrinsicElements` interface:

```typescript
// hovering over `something` shows:

Property 'something' does not exist on type 'JSX.IntrinsicElements'.
```

It's not the code that's incorrect, it's the types that are insufficient.

## Challenge

Your goal is to strongly type the `something` element so that it can only be used with the correct attributes.

Hint: You'll need to use declaration merging with something in the global namespace that we've seen before.

By utilizing declaration merging, you can inform TypeScript of the types of custom elements from external libraries.

For the solution to this challenge, we'll name the element `<something-solution>`.

From the error message, we know that the `something-solution` element does not exist on the type `JSX.IntrinsicElements`. 

This tells us that we need to use declaration merging to extend the `JSX.IntrinsicElements` interface.

Like we've seen before, we'll use `declare global` and target the `JSX` namespace. Then we'll extend the `IntrinsicElements` interface with the new `something-solution` element:

```typescript
declare global {
  namespace JSX {
    interface IntrinsicElements {
      "something-solution": {
        id: string;
      };
    }
  }
}
```

After extending the interface the error disappears.

When using the `something-solution` element, we have to pass a string for the `id` prop.

As seen in the tests, not passing the `id` prop or passing a number for the `id` prop will throw an error:

If we pass the wrong type of prop, TypeScript will throw an error, ensuring that we only pass the correct prop.

```tsx

<something-solution />
// Error: Property 'id' is missing in type '{}' but required in type '{ id: string; }'.

<something-solution id={123} />
// Error: Type 'number' is not assignable to type 'string'.
```

## Alternative Solution
Recall that `JSX.IntrinsicElements` is available both in the global `JSX` namespace and in the `React` namespace. 

The first solution uses the global `JSX` namespace, but we could also use the `React.JSX` namespace.

```typescript
declare global {
  namespace React{
    namespace JSX {
      interface IntrinsicElements {
        "something-solution": {
          id: string;
        };
      }
  }}
}
```

While both of these options will work, it's generally recommended to use `React.JSX.IntrinsicElements` when working with React because it's more specific.

Type external JSX elements through declaration merging by extending JSX.IntrinsicElements.

Extend JSX.IntrinsicElements with Declaration Merging

[00:00] So the thing we know from this error is that JSX.IntrinsicElements is the thing we want to target with our declaration merge. We want to pull this into the global scope here, so we'll declare global. To declare global, we're going to basically say,

[00:16] we need to go onto the namespace JSX and inside there, we're going to go interface intrinsic elements. I have to make sure to spell this right, intrinsic elements, and then say something solution, because it has to be inside a string here, otherwise it's invalid,

[00:33] and we're going to say ID string. Now here, it starts to work and we actually get, you can see here, there we go, JSX.IntrinsicElements something solution ID string. If we command click, we actually go to this definition up here, whereas if we were to command click on a div, let's say, we would actually go all the way

[00:51] into JSX.IntrinsicElements here. You're starting to see how declaration merging can be pretty powerful here, because we can actually describe custom stuff that we can render, which is really, really interesting. So now, if we pass in like the wrong thing, if we don't pass in an ID, it's missing in type empty object, but required in this,

[01:09] and also if we pass it as the wrong thing, this is going to yell at us because it's expecting a string. Very, very cool. And you notice as well, that JSX.IntrinsicElements, it's also available inside the namespace React, JSX.IntrinsicElements. So up here, for instance, this also will work.

[01:29] This is interesting because JSX.IntrinsicElements, it's available on the JSX namespace, but that's global. And if for whatever reason, you have types that are coming in from, let's say, React, but also another framework that uses JSX, like SolidJS or Preact, for instance, you actually want to say,

[01:48] okay, this is only available inside React's namespace of JSX. So it gets a little bit confusing here, but really the main thing is that both of these will work. And probably you want to be more specific, I would say. So you probably want to say React.JSX.IntrinsicElements. But then again, how often are you going to be combining

[02:06] two frameworks in the same project? So I would usually just say, namespace JSX.IntrinsicElements, add your extra kind of elements on there and type the props. Not bad.

Add a New Global Element in TypeScript

[00:00] In this exercise, I want you to imagine that we're working with an external library that has a new global element available to us. We want to strongly type this so that it can only be used with the correct attributes. In this case, we want to create a something element and give it a required ID attribute, which is a string.

[00:17] We're going to need to declaration merge with something in the global namespace that we've seen before. I think you're getting the idea here. In fact, the error that we're getting here is property something does not exist on type JSX.IntrinsicElements. In this case, we're not wrong, it's the types that are wrong.

[00:37] Learning what you've figured out so far, see if you can piece all that together and make this work. Good luck.

In this Advanced React with TypeScript workshop, you’ll learn how to combine the most important concepts and patterns of React with TypeScript. 

This combination is a superpower that will change the way you and your team build web applications.

You’ll write custom hooks that use generics, share some (but not all) props between components, and remove guesswork by diving into the types behind the scenes of React and other popular libraries.

## Prerequisites

You'll need a working knowledge of React, as well as foundational TypeScript knowledge. Working through the [React with TypeScript Tutorial](https://www.totaltypescript.com/tutorials/react-with-typescript) is recommended.





Expert-level knowledge for typing your React apps with TypeScript.

React TypeScript

Total TypeScript Complete Volume is a set of five full TypeScript workshops for the advanced-beginner to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle includes all of the Total TypeScript self-paced workshops.

TypeScript Pro Essentials

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

TypeScript Pro Complete

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Solving Partial Autocompletion

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Understanding and Implementing Dynamic Props Mapping in React

Inference from a Single Source of Truth

Advanced Props

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Refactor complex type logic into a type helper.

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Refactoring to a Type Helper

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Enforcing constraints helps you prevent errors from occurring, and ensures your type helpers are being used in the correct way.

Add Generic Constraints to Type Helpers

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By default, type arguments are inferred as `unknown`. When using this type of API in your code, it is crucial to have it well-documented.

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Type Inference with Generic Functions in TypeScript

Wrapping a Generic Function Inside of Another

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By adding a generic type argument to a props interface, we can infer prop types.

Add a Generic Type Argument to a Props Interface

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Add a New Global Element in TypeScript

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