[00:00] Here, we're back with our configurations object, which has development, production, and staging members of that object. We are basically saying type environment is key of type of configurations. We have development and production and staging being inferred from this. But we're expecting an error when we pass

[00:18] something to this configurations object, which is not supposed to be there. Your job is to try to figure out how we can annotate configurations so that we still get environment properly inferred, but we get errors on this not allowed member on this value. Good luck.


We're back to the `configurations` object that includes `development`, `production`, and `staging`. Each of these members contains specific settings relevant to its environment:

```tsx
const configurations = {
  development: {
    apiBaseUrl: "http://localhost:8080",
    timeout: 5000,
  },
  production: {
    apiBaseUrl: "https://api.example.com",
    timeout: 10000,
  },
  staging: {
    apiBaseUrl: "https://staging.example.com",
    timeout: 8000,
    // @ts-expect-error // red squiggly line under @ts-expect-error
    notAllowed: true,
  },
};
```

Inside of `staging` we're expecting an error on `notAllowed: true`, but the `@ts-expect-error` directive is not working as expected.

We have an `Environment` type along with a test case that checks if `Environment` is equal to `"development" | "production" | "staging"`:

```typescript
type Environment = keyof typeof configurations;

type test = Expect<
  Equal<Environment, "development" | "production" | "staging">
>;

```

## Challenge

The test passes, but we want there to be an error when attempting to include something into the `configurations` object that should not be there.

Your task is to determine an appropriate way to annotate our `configurations` object to retain accurate `Environment` inference from it while simultaneously throwing an error for disallowed members.


The satisfies operator allows you to retain inference while ensuring that objects meet certain criteria.

[00:00] Okay. First of all, we need to think about what shape we want configurations to be. We probably want it to be a record of string, and then the value of that is going to be API base URL, lovely, which is a string, and then timeout, which is a number.

[00:16] Beautiful. And now we're getting an error because not allowed is not allowed in that situation. But there's another error happening below because environment is now typed as string. Because if we think about the configurations variable, this is not being inferred as the thing being passed to it. It's actually just being typed.

[00:35] So we have configurations. This is now the key here, type of key of this configurations is record string, or rather, it's just string. So how can we get around this? We need to make sure that configurations is still being inferred as its type, but we need type checking on the thing being passed to it. Well, let's add satisfies.

[00:54] Satisfies is perfect for this situation. Because it means that we can say this satisfies this criteria while still having these types inferred. So development, production, and staging are still now in the type. So satisfies is really useful when you're doing key of type of tricks because it means that you can still get the inference,

[01:14] still get the single source of truth from the value that you're adding there without needing to override it. Beautiful, beautiful stuff. So satisfies, again, if you're ever declaring a config object, satisfies is probably what you're going to be needing to use.


The first step is to determine the structure of our `configurations` object.

In this case, it makes sense for it to be a `Record` where the keys will be `string` and the values will be an object with `apiBaseUrl` and `timeout` properties.

```typescript
const configurations: Record<
  string,
  {
    apiBaseUrl: string;
    timeout: number
  }
> = {
  ...
``` 

This change makes the `@ts-expect-error` directive work as expected, but we now have an error related to the `Environment` type not being inferred correctly:

```typescript
type Environment = keyof typeof configurations;

// hovering over Environment shows:
// type Environment = string

// The test now fails:
type test = Expect<
  Equal<Environment, "development" | "production" | "staging"> // red squiggly line under Equal<>
```

We need to make sure that `configurations` is still being inferred as its type, while also type checking the thing being passed to it.

This is the perfect application for the `satisfies` keyword.

Instead of annotating the `configurations` object as a `Record`, we'll instead use the `satisfies` keyword for the type constraint:

```typescript
const configurations = {
  development: {
    apiBaseUrl: "http://localhost:8080",
    timeout: 5000,
  },
  production: {
    apiBaseUrl: "https://api.example.com",
    timeout: 10000,
  },
  staging: {
    apiBaseUrl: "https://staging.example.com",
    timeout: 8000,
    // @ts-expect-error
    notAllowed: true,
  },
} satisfies Record<
  string,
  {
    apiBaseUrl: string;
    timeout: number;
  }
>;
```

This allows us to specify that the values we pass to our configuration object must adhere to the criteria defined in the type, while still allowing the type system to infer the correct types for our development, production, and staging environments.

Using of `satisfies` is a good practice when using objects like these to maintain strong type checking while leaving room for inference.

Using the satisfies operator with a Record allows you to specify type constraints without affecting type inference.

Specify Type Constraints with the satisfies Operator

Using satisfies with keyof and typeof in TypeScript

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Unexpected Token Error in the Browser

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Compile TypeScript to JavaScript with the TypeScript CLI

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Compiling TypeScript Files to a Directory

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Use Vite for a Frontend App with TypeScript

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Comparing Vite with tsc

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Quickly Create Scripts with tsx

TypeScript in The Build Process

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Learn about the two different syntaxes for defining arrays in TypeScript.

Two Syntaxes for Arrays

Add an Array to an Object

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There are few different ways in which an array of objects can be expressed

Representing an Array of Objects

Arrays of Objects in TypeScript

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Rest Parameters in TypeScript

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Tuples for Precise Array Structures in TypeScript

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Function Types for Type Aliases in TypeScript

Function Types

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The `void` Type

Typing an Event Listener

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TSDoc Comments

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Literal Type Assignability

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Using satisfies with keyof and typeof in TypeScript

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