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.