Type assertions in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code with type assertion?

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

Typescript

let someValue: unknown = "Hello TypeScript!";
let strLength: number = (someValue as string).length;
console.log(strLength);

A Compilation error

B undefined

C 17

D TypeError at runtime

💡 Hint

Type assertion tells TypeScript to treat a value as a specific type.

The variable someValue is asserted as a string, so accessing .length works and returns 17, the length of "Hello TypeScript!".

❓ Predict Output

intermediate

What error does this TypeScript code raise?

What error will this TypeScript code produce when compiled or run?

Typescript

let value: any = 123;
let strLength: number = (value as string).length;
console.log(strLength);

A Runtime error: Cannot read property 'length' of number

B Outputs undefined

C Compilation error: Cannot assert number to string

D Outputs 3

💡 Hint

Type assertion does not change the actual runtime type.

At runtime, value is a number, so accessing .length causes a runtime error.

🧠 Conceptual

advanced

Which option correctly uses type assertion to access a property?

Given an object of unknown type, which option correctly asserts it as a specific type to access its property without error?

A

let obj: unknown = {name: "Alice"};
console.log((obj as {name: string}).name);

B

let obj: unknown = {name: "Alice"};
console.log(obj.name);

C

let obj: unknown = {name: "Alice"};
console.log(<{name: string}>obj.name);

D

let obj: unknown = {name: "Alice"};
console.log(obj as name);

💡 Hint

Type assertion syntax uses 'as' followed by the type in parentheses.

Option A correctly asserts obj as an object with a name property of type string, allowing access to .name.

📝 Syntax

advanced

Which option has correct syntax for type assertion in TypeScript?

Which of the following is the correct syntax for asserting a variable 'x' as type 'number'?

A let y = x <number>;

B let y = x : number;

C let y = <number> x;

D let y = x as number;

💡 Hint

Use 'as' keyword for type assertion in modern TypeScript.

Option D uses the modern and recommended 'as' syntax for type assertion.

🚀 Application

expert

What is the value of 'result' after running this TypeScript code?

Analyze the code below and determine the value of 'result' after execution.

Typescript

interface User {
  id: number;
  name: string;
}

const data: unknown = {id: 42, name: "Bob"};
const user = data as User;
const result = user.id + user.name.length;

A Compilation error

B 45

C TypeError at runtime

D NaN

💡 Hint

Type assertion does not change the actual runtime value.

The object has id 42 and name "Bob" (length 3), so result is 42 + 3 = 45.