Why Readonly properties in interfaces in Typescript? - Purpose & Use Cases

The Big Idea

What if a simple keyword could protect your data from accidental changes forever?

The Scenario

Imagine you are building a form where users enter their profile data. You want to make sure some fields, like user ID, never change after creation. Without readonly properties, you have to constantly check and prevent accidental changes manually.

The Problem

Manually tracking which properties should not change is slow and error-prone. You might forget to add checks everywhere, leading to bugs where important data is accidentally modified. This makes your code fragile and hard to maintain.

The Solution

Readonly properties in interfaces let you declare which fields cannot be changed after they are set. The TypeScript compiler then helps you by giving errors if you try to modify those fields, keeping your data safe automatically.

Before vs After

✗ Before

interface User { id: number; name: string; }
const user: User = { id: 1, name: 'Alice' };
user.id = 2; // Oops, no error, but should not change

✓ After

interface User { readonly id: number; name: string; }
const user: User = { id: 1, name: 'Alice' };
user.id = 2; // Error: Cannot assign to 'id' because it is a read-only property

What It Enables

This lets you write safer code by clearly marking data that should never change, preventing bugs before they happen.

Real Life Example

In a banking app, account numbers should never change once assigned. Using readonly properties ensures these critical values stay constant throughout the app.

Key Takeaways

Readonly properties prevent accidental changes to important data.

They help catch errors early during development.

Using them makes your code more reliable and easier to maintain.

Practice

1.

What does the readonly keyword do when used in a TypeScript interface property?

easy

A. Prevents the property from being changed after initialization

B. Allows the property to be changed anytime

C. Makes the property optional

D. Makes the property private

Solution

1. Step 1: Understand the purpose of readonly in interfaces

   The readonly keyword marks a property so it cannot be reassigned after the object is created.

2. Step 2: Compare with other options

   Options A, B, and D describe different behaviors: A makes optional, B allows changes, D changes visibility, none of which match readonly.

3. Final Answer:

   Prevents the property from being changed after initialization -> Option A

4. Quick Check:

   readonly means no changes allowed after creation [OK]

Hint: Readonly means no changes allowed after creation [OK]

Common Mistakes:

• Thinking readonly makes property optional
• Confusing readonly with private or protected
• Assuming readonly allows reassignment

2.

Which of the following is the correct syntax to declare a readonly property id of type number in a TypeScript interface?

interface User {
  ?
}

easy

A. id: readonly number;

B. id readonly: number;

C. readonly: id number;

D. readonly id: number;

Solution

1. Step 1: Recall correct readonly syntax in interfaces

   The correct syntax places readonly before the property name, then a colon and the type.

2. Step 2: Check each option

   readonly id: number; matches the correct syntax. Options A, B, and C have incorrect order or misplaced keywords.

3. Final Answer:

   readonly id: number; -> Option D

4. Quick Check:

   Readonly keyword comes before property name [OK]

Hint: Put readonly before property name in interface [OK]

Common Mistakes:

• Placing readonly after property name
• Using readonly as a type
• Misplacing colons or semicolons

3.

Consider the following code:

interface Point {
  readonly x: number;
  readonly y: number;
}

const p: Point = { x: 10, y: 20 };
p.x = 30;
console.log(p.x);

What will happen when this code runs?

medium

A. It will print 30

B. It will print 10

C. Compile-time error due to assignment to readonly property

D. Runtime error when assigning to readonly property

Solution

1. Step 1: Understand readonly behavior at compile time

   TypeScript prevents assigning a new value to a readonly property during compilation, causing an error before running.

2. Step 2: Check runtime behavior

   Since TypeScript errors stop compilation, the code won't run, so no runtime error or output occurs.

3. Final Answer:

   Compile-time error due to assignment to readonly property -> Option C

4. Quick Check:

   Readonly blocks assignment at compile time [OK]

Hint: Readonly errors happen at compile time, not runtime [OK]

Common Mistakes:

• Thinking readonly allows reassignment at runtime
• Expecting runtime error instead of compile error
• Assuming original value changes silently

4.

Find the error in this code snippet:

interface Config {
  readonly url: string;
}

const config: Config = { url: "https://api.com" };
config.url = "https://newapi.com";

medium

A. Cannot assign to 'url' because it is a readonly property

B. Missing semicolon after interface declaration

C. Type mismatch: string assigned to readonly property

D. Interface properties cannot be initialized

Solution

1. Step 1: Identify the readonly property usage

   The property url is marked readonly, so it cannot be reassigned after initialization.

2. Step 2: Check the assignment line

   The line config.url = "https://newapi.com"; tries to change a readonly property, causing a compile-time error.

3. Final Answer:

   Cannot assign to 'url' because it is a readonly property -> Option A

4. Quick Check:

   Readonly properties cannot be reassigned [OK]

Hint: Readonly properties cause errors if reassigned [OK]

Common Mistakes:

• Ignoring readonly and trying to reassign
• Confusing readonly with optional properties
• Thinking semicolon or type is the error

5.

You want to create an interface Person with a readonly id and a mutable name. Which code correctly enforces this?

?

hard

A. interface Person { id: readonly number; name: string; }

B. interface Person { readonly id: number; name: string; }

C. interface Person { readonly id: number; readonly name: string; }

D. interface Person { id: number; name: readonly string; }

Solution

1. Step 1: Define readonly and mutable properties correctly

   The readonly keyword must be before the property name to make it immutable; properties without it remain mutable.

2. Step 2: Evaluate each option

   interface Person { readonly id: number; name: string; } correctly marks id as readonly and name as mutable. Options B, C, and D misuse readonly or make both properties readonly.

3. Final Answer:

   interface Person { readonly id: number; name: string; } -> Option B

4. Quick Check:

   Readonly before property name only for immutable fields [OK]

Hint: Put readonly before only the properties you want fixed [OK]

Common Mistakes:

• Placing readonly after property name or type
• Making all properties readonly by mistake
• Confusing readonly with type annotations