Read-only arrays in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code using a readonly array?

Consider the following TypeScript code. What will be printed to the console?

Typescript

const nums: readonly number[] = [1, 2, 3];
// nums.push(4); // This line is commented out
console.log(nums[1]);

A Error: Cannot read property

B undefined

C 2

D 3

💡 Hint

Readonly arrays allow reading elements but not modifying them.

The readonly array nums contains [1, 2, 3]. Accessing nums[1] returns 2. The push method is not called because it is commented out.

❓ Predict Output

intermediate

What error does this code raise when trying to modify a readonly array?

What error will this TypeScript code produce when compiled?

Typescript

const letters: readonly string[] = ['a', 'b', 'c'];
letters[0] = 'z';

A Error: Index signature in type 'readonly string[]' only permits reading

B TypeError at runtime

C SyntaxError

D No error, array modified

💡 Hint

Readonly arrays prevent assignment to elements.

TypeScript compiler will show an error because you cannot assign a value to an index of a readonly array.

🔧 Debug

advanced

Why does this code fail to compile when using readonly arrays?

Identify the reason why this TypeScript code does not compile.

Typescript

function addItem(arr: readonly number[], item: number) {
  arr.push(item);
  return arr;
}

A Readonly arrays do not have the push method

B Function parameters cannot be readonly

C Cannot return arrays from functions

D Missing return type annotation

💡 Hint

Readonly arrays do not allow modification methods.

Readonly arrays do not have mutating methods like push, so calling push causes a compile error.

🧠 Conceptual

advanced

Which statement about readonly arrays in TypeScript is true?

Choose the correct statement about readonly arrays.

A Readonly arrays are mutable but cannot be reassigned to a new array

B Readonly arrays can be modified using map or filter methods

C Readonly arrays allow push but not pop

D Readonly arrays prevent any modification including element reassignment and push/pop

💡 Hint

Think about what readonly means for arrays.

Readonly arrays prevent any modification to their elements or structure, including reassignment of elements and use of mutating methods like push or pop.

❓ Predict Output

expert

What is the output of this code mixing readonly and mutable arrays?

What will be the output of this TypeScript code?

Typescript

const original: number[] = [1, 2, 3];
const readOnlyView: readonly number[] = original;
original.push(4);
console.log(readOnlyView.length);

A Error: Cannot push to original

B 4

C 3

D Error: Cannot assign original to readonly array

💡 Hint

Readonly arrays are views, not copies.

readOnlyView references the same array as original. Pushing to original changes the array length, so readOnlyView.length is 4.

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