Readonly properties in Typescript - Step-by-Step Execution

Concept Flow - Readonly properties

Define object with readonly property

↓

Try to assign new value to readonly property

This flow shows that once a readonly property is set, trying to change it causes an error and stops the program.

Execution Sample

Typescript

interface Person {
  readonly name: string;
}

const p: Person = { name: "Alice" };
p.name = "Bob"; // Error

This code defines a readonly property 'name' and then tries to change it, which causes an error.

Execution Table

Step	Action	Property 'name' Value	Result
1	Create object p with name = 'Alice'	'Alice'	Object created successfully
2	Attempt to assign p.name = 'Bob'	'Alice'	Error: Cannot assign to 'name' because it is a read-only property

💡 Execution stops at step 2 due to assignment error on readonly property

Variable Tracker

Variable	Start	After Step 1	After Step 2
p.name	undefined	'Alice'	'Alice' (unchanged due to error)

Key Moments - 2 Insights

Why can't we change the value of a readonly property after the object is created?

Readonly properties are designed to prevent changes after initialization. As shown in execution_table step 2, trying to assign a new value causes a compile-time error, so the value stays the same.

Does readonly mean the property cannot be set at all?

No, readonly means the property can be set once when the object is created (step 1), but cannot be changed later (step 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of p.name after step 2?

A 'Bob'

B 'Alice'

C undefined

D null

💡 Hint

Check the 'Property 'name' Value' column at step 2 in execution_table

At which step does the error occur when trying to change the readonly property?

A Step 1

B No error occurs

C Step 2

D After execution ends

💡 Hint

Look at the 'Result' column in execution_table for the error message

If we remove 'readonly' from the property, what would happen at step 2?

A Value changes to 'Bob' successfully

B Error still occurs

C Object creation fails

D Property becomes undefined

💡 Hint

Think about how readonly affects assignment shown in execution_table

Concept Snapshot

Readonly properties in TypeScript:
- Use 'readonly' keyword in interfaces or classes
- Property can be set once during object creation
- Cannot be changed later; assignment causes error
- Helps protect data from accidental changes
- Compile-time error if reassigned

Full Transcript

This visual execution shows how readonly properties work in TypeScript. First, an object with a readonly property 'name' is created and set to 'Alice'. Then, an attempt to change 'name' to 'Bob' causes a compile-time error, so the value remains 'Alice'. Readonly means you can set the property once but cannot change it later. This prevents accidental changes to important data.

Practice

1. What does the readonly keyword do in TypeScript?

easy

A. Prevents a property from being changed after it is set

B. Allows a property to be changed anytime

C. Makes a property private and inaccessible

D. Deletes a property from an object

Solution

1. Step 1: Understand the purpose of readonly

   The readonly keyword is used to mark properties that cannot be changed after initialization.

2. Step 2: Compare options with the behavior

   Only Prevents a property from being changed after it is set correctly describes that the property cannot be changed after it is set.

3. Final Answer:

   Prevents a property from being changed after it is set -> Option A

4. Quick Check:

   readonly means no changes allowed after set [OK]

Hint: Readonly means no changes allowed after initial set [OK]

Common Mistakes:

• Thinking readonly allows changes anytime
• Confusing readonly with private
• Assuming readonly deletes properties

2. Which of the following is the correct way to declare a readonly property id in a TypeScript class?

easy

A. const id: number;

B. readonly id: number;

C. private readonly id: number;

D. id readonly: number;

Solution

1. Step 1: Recall TypeScript syntax for readonly properties

   The correct syntax is to place readonly before the property name.

2. Step 2: Check each option

   readonly id: number; uses readonly id: number; which is correct. const id: number; uses const which is invalid inside class properties. private readonly id: number; adds private which is valid but not asked for. id readonly: number; has wrong order.

3. Final Answer:

   readonly id: number; -> Option B

4. Quick Check:

   Readonly keyword goes before property name [OK]

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

Common Mistakes:

• Using const instead of readonly in class
• Placing readonly after property name
• Confusing access modifiers with readonly

3. What will be the output of this TypeScript code?

class User {
  readonly name: string;
  constructor(name: string) {
    this.name = name;
  }
}
const user = new User('Alice');
user.name = 'Bob';
console.log(user.name);

medium

A. Alice

B. Bob

C. undefined

D. Compile-time error

Solution

1. Step 1: Understand readonly property behavior

   The property name is marked readonly, so it cannot be assigned a new value after construction.

2. Step 2: Analyze the assignment user.name = 'Bob'

   This line tries to change a readonly property, which causes a compile-time error in TypeScript.

3. Final Answer:

   Compile-time error -> Option D

4. Quick Check:

   Assigning to readonly property causes error [OK]

Hint: Changing readonly property causes compile error [OK]

Common Mistakes:

• Assuming readonly properties can be changed anytime
• Expecting runtime error instead of compile error
• Thinking output will be the new value

4. Identify the error in this TypeScript code and fix it:

interface Point {
  readonly x: number;
  y: number;
}
const p: Point = { x: 10, y: 20 };
p.x = 15;
console.log(p.x);

medium

A. Property 'y' should be readonly too

B. Missing type annotation for 'p'

C. Cannot assign to 'x' because it is a readonly property

D. No error, code runs fine

Solution

1. Step 1: Check readonly property assignment

   The property x is readonly in the interface, so assigning p.x = 15 is not allowed.

2. Step 2: Identify the error message

   TypeScript will report an error: "Cannot assign to 'x' because it is a readonly property".

3. Final Answer:

   Cannot assign to 'x' because it is a readonly property -> Option C

4. Quick Check:

   Readonly properties cannot be assigned after initialization [OK]

Hint: Readonly properties cannot be reassigned after creation [OK]

Common Mistakes:

• Ignoring readonly and trying to assign anyway
• Thinking missing type annotation causes error
• Assuming no error occurs

5. You want to create a TypeScript class Config where the version property is set once during construction and never changes, but settings can be updated anytime. Which is the best way to declare these properties?

hard

A. readonly version: string; settings: object;

B. version: string; readonly settings: object;

C. readonly version: string; readonly settings: object;

D. version: string; settings: object;

Solution

1. Step 1: Understand property requirements

   The version should be readonly (set once, no changes). The settings should be mutable (can change anytime).

2. Step 2: Match options with requirements

   readonly version: string; settings: object; correctly marks version as readonly and leaves settings mutable. version: string; readonly settings: object; reverses roles. readonly version: string; readonly settings: object; makes both readonly. version: string; settings: object; makes both mutable.

3. Final Answer:

   readonly version: string; settings: object; -> Option A

4. Quick Check:

   Readonly for fixed, mutable for changeable properties [OK]

Hint: Readonly for fixed, normal for changeable properties [OK]

Common Mistakes:

• Making mutable properties readonly by mistake
• Not using readonly for fixed properties
• Confusing which property should be readonly