[00:00] Let's say that we have an update user function which takes in a user of type user. Now, this type user has three properties, ID, name, and age. And inside the update user function, we want to be able to modify the name and the age, but we don't want to be able to modify the ID. The ID should always stay static.

[00:18] Your job is to work out how to change this type definition of user to give ID a read-only attribute. Good luck.


Here we have an `updateUser` function which accepts a `user` of type `User`.

The `User` type has three properties: `id`, `name`, and `age`. 

Inside the `updateUser` function, our goal is to allow modifications to the `name` and `age` properties, while ensuring that the `id` remains constant and cannot be changed.

```typescript
type User = {
  id: number;
  name: string;
  age: number;
}

const updateUser = (user: User) => {
    user.name = "Jane Doe";
    user.age = 30;

    // @ts-expect-error Should not be able to modify readonly // red squiggly line under full line
    user.id = 1;
}
```

## Challenge

Your challenge is to figure out how to modify the type definition for `User` to make `id` a read-only property. This will prevent the `id` from being changed, and the error will go away.

At times, we want to prevent properties from being modified. TypeScript offers a way to do this with read-only properties.

[00:00] Okay, the solution here is pretty simple. TypeScript gives us a first class way to do that. We can just add read-only before the ID property, just there. And now the ID property is read-only and the rest of them are still mutable. So by default, object properties are mutable in TypeScript. And this is because by default,

[00:19] object properties are mutable in JavaScript too. Adding read-only here doesn't do anything at runtime, right? It's just literally a type annotation, but it just gives you a bit more information and says, okay, if you try to mutate this, you can't do it because it's a read-only property. And I think a lot more people should be using read-only in their type definitions,

[00:38] because how often really are you mutating objects directly? Usually in the kind of front-end or very immutable world that we live in, immutability is a key concept in front-end applications and back-end applications. Really, you want a lot of your properties to be read-only. So I think you should probably be using this a fair amount.

[00:57] But anyway, it's a very, very useful little type annotation that can really save you from quite a lot of bugs.


The solution to this challenge is pretty straightforward.

TypeScript offers a first-class approach for this by adding the `readonly` keyword before the property, like so: 

```typescript
type User = {
  readonly id: number;
  name: string;
  age: number;
};
```

With this change, the `id` property becomes read-only, whereas the other properties `name` and `age` remain mutable.

Remember, object properties in both TypeScript and JavaScript are mutable by default.

Adding of `readonly` to a property has no runtime effect, but instead is a type annotation that provides crucial information. It informs you that a certain property should not be changed once it is set, which eliminates the risk of unwarranted mutations.

The `readonly` annotation is a tool that more developers should adopt in their type definitions.

In both front-end and back-end applications, where immutability is a foundational concept, direct mutation of objects is often avoided. Therefore, many properties should ideally be set as read-only. 

The `readonly` type annotation is a simple but remarkably handy feature that can protect your code from bugs.

TypeScript's readonly modifier can be used to prevent object properties from being modified.

Specifying Read-only Object Properties

Creating Read-only Properties

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Unexpected Token Error in the Browser

Use the TypeScript CLI to convert a TypeScript file into a JavaScript file.

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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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Create a new project with Vite and TypeScript to improve upon our current setup.

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Use Vite for a Frontend App with TypeScript

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Comparing Vite with tsc

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The tsx CLI tool allows you to quickly run scripts written in TypeScript.

Quickly Create Scripts with tsx

TypeScript in The Build Process

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Add a Default Parameter with `=`

Default Parameters in JavaScript Functions

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Annotating with {}

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Making Object Property Types Optional

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Two Syntaxes for Arrays

Add an Array to an Object

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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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How to make sure a function and its parameters and return types fit certain criteria, and how to annotate functions passed as arguments

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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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Typing Fetch API Responses in Async Functions

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Understanding How TypeScript Works in Your IDE

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Hovering Over a Function Call

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TSDoc Comments

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Manually Triggering Autocomplete

TypeScript's Approach to Errors

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The Organize Imports Hotkey in VS Code

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Automatic Code Formatting with Prettier

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Introducing Union Types in TypeScript

Handling Null Values in TypeScript

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Combining Literal & Union Types

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Literal types are assignable based on their literal values.

Literal Type Assignability

Learn how to combine unions in TypeScript to create type aliases with as little duplication as possible.

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Creating Read-only Properties

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