Read-only arrays in Typescript - Mini Project: Build & Apply

Working with Read-only Arrays in TypeScript

📖 Scenario: Imagine you are managing a list of favorite fruits that should not be changed accidentally in your program. You want to make sure this list stays the same throughout your code.

🎯 Goal: You will create a read-only array of fruits, then try to work with it safely without changing the original list.

📋 What You'll Learn

Create a read-only array of strings with exact fruits

Create a variable to hold the number of fruits

Use a for loop to list all fruits

Print the list of fruits

💡 Why This Matters

🌍 Real World

Read-only arrays help protect important lists of data from accidental changes, like a fixed menu or a list of constants.

💼 Career

Understanding read-only arrays is useful for writing safer TypeScript code in jobs that require data integrity and immutability.

1

Create a read-only array of fruits

Create a read-only array called fruits with these exact values: 'apple', 'banana', 'cherry', 'date'.

Typescript

# Your code here

Hint

Use readonly string[] to make the array read-only.

2

Create a variable for the number of fruits

Create a variable called count and set it to the length of the fruits array.

Typescript

const fruits: readonly string[] = ['apple', 'banana', 'cherry', 'date'];
# Your code here

Hint

Use the length property of the array.

3

Use a for loop to list all fruits

Use a for loop with variable i from 0 to count - 1 to access each fruit in fruits.

Typescript

const fruits: readonly string[] = ['apple', 'banana', 'cherry', 'date'];
const count = fruits.length;
# Your code here

Hint

Use a classic for loop to go through the array by index.

4

Print the list of fruits

Print the entire fruits array using console.log.

Typescript

const fruits: readonly string[] = ['apple', 'banana', 'cherry', 'date'];
const count = fruits.length;
for (let i = 0; i < count; i++) {
    console.log(fruits[i]);
}
# Your code here

Hint

Use console.log(fruits) to print the whole array.

Practice

1. What does a readonly array in TypeScript do?

easy

A. Only prevents changing the array length

B. Allows adding new elements but not removing existing ones

C. Makes the array elements optional

D. Prevents any changes to the array elements after creation

Solution

1. Step 1: Understand the readonly keyword

   The readonly keyword before an array type means the array cannot be changed after it is created.

2. Step 2: Effects on the array

   This means you cannot add, remove, or modify elements in the array once it is set.

3. Final Answer:

   Prevents any changes to the array elements after creation -> Option D

4. Quick Check:

   readonly arrays are immutable [OK]

Hint: Readonly arrays block all changes to elements [OK]

Common Mistakes:

• Thinking readonly only blocks length changes
• Assuming elements can be modified individually
• Confusing readonly with optional elements

2. Which of the following is the correct syntax to declare a read-only array of numbers in TypeScript?

easy

A. let nums: readonly number[] = [1, 2, 3];

B. let nums: number[] readonly = [1, 2, 3];

C. let nums: number readonly[] = [1, 2, 3];

D. let nums: number[] = readonly [1, 2, 3];

Solution

1. Step 1: Recall the correct readonly syntax

   In TypeScript, readonly comes before the array type, like readonly number[].

2. Step 2: Check each option

   let nums: readonly number[] = [1, 2, 3]; uses readonly number[], which is correct syntax. Others place readonly incorrectly.

3. Final Answer:

   let nums: readonly number[] = [1, 2, 3]; -> Option A

4. Quick Check:

   readonly before type [OK]

Hint: Put readonly before the array type, not after [OK]

Common Mistakes:

• Placing readonly after the array brackets
• Using readonly as a variable modifier
• Trying to put readonly inside the array literal

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

const arr: readonly string[] = ['a', 'b', 'c'];
// arr.push('d');
console.log(arr.length);

medium

A. 4

B. undefined

C. 3

D. Error: Property 'push' does not exist on type 'readonly string[]'

Solution

1. Step 1: Understand the array declaration

   The array arr is declared as readonly string[] with 3 elements: 'a', 'b', 'c'.

2. Step 2: Check the commented out push and output

   The push line is commented out, so it does not run. The length remains 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Readonly array length unchanged = 3 [OK]

Hint: Commented push means length stays same [OK]

Common Mistakes:

• Assuming push works on readonly arrays
• Expecting length to increase despite push commented
• Confusing runtime errors with compile-time errors

4. Identify the error in this TypeScript code using a read-only array:

const nums: readonly number[] = [1, 2, 3];
nums[0] = 10;

medium

A. Type 'number' is not assignable to type 'readonly number[]'

B. Cannot assign to '0' because it is a read-only property

C. Syntax error: missing semicolon

D. No error, code runs fine

Solution

1. Step 1: Understand the readonly array behavior

   The array nums is readonly, so its elements cannot be changed after creation.

2. Step 2: Analyze the assignment to an element

   Trying to assign nums[0] = 10 causes an error because elements are read-only.

3. Final Answer:

   Cannot assign to '0' because it is a read-only property -> Option B

4. Quick Check:

   Readonly elements cannot be changed [OK]

Hint: Readonly arrays block element assignment [OK]

Common Mistakes:

• Thinking only array structure is readonly, not elements
• Ignoring TypeScript error messages
• Assuming semicolon missing causes error

5. Given a function that accepts a read-only array, which of these operations inside the function is allowed?

function process(items: readonly number[]) {
  // Which line is valid?
  // A) items.push(5);
  // B) items[0] = 10;
  // C) const first = items[0];
  // D) items.length = 0;
}

hard

A. const first = items[0];

B. items.push(5);

C. items[0] = 10;

D. items.length = 0;

Solution

1. Step 1: Understand read-only array restrictions

   Read-only arrays do not allow modifying methods like push, element assignment, or length changes.

2. Step 2: Check each operation

   Only reading elements, like const first = items[0];, is allowed. Others cause errors.

3. Final Answer:

   const first = items[0]; -> Option A

4. Quick Check:

   Reading elements allowed, modifying disallowed [OK]

Hint: Readonly arrays allow reading, not modifying [OK]

Common Mistakes:

• Trying to push or assign elements in readonly arrays
• Attempting to change array length
• Confusing read-only with mutable arrays