Type alias for functions in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript function using a type alias?

Consider the following TypeScript code that uses a type alias for a function. What will be printed when greet('Alice') is called?

Typescript

type Greet = (name: string) => string;

const greet: Greet = (name) => {
  return `Hello, ${name}!`;
};

console.log(greet('Alice'));

A "Hello, Alice!"

B Error: greet is not a function

C undefined

D Hello, Alice!

💡 Hint

Remember that console.log prints the string without quotes.

The function greet returns the string Hello, Alice!. When printed with console.log, the output is Hello, Alice! without quotes.

🧠 Conceptual

intermediate

Which type alias correctly describes a function that takes two numbers and returns a boolean?

Choose the correct TypeScript type alias for a function that takes two number arguments and returns a boolean.

A type Compare = (a: string, b: string) => boolean;

B type Compare = (a: number, b: number) => number;

C type Compare = (a: number, b: number) => boolean;

D type Compare = (a: number) => boolean;

💡 Hint

Check the parameter types and the return type carefully.

The function must take two numbers and return a boolean. Option C matches this exactly.

🔧 Debug

advanced

What error does this TypeScript code produce?

Examine the following TypeScript code using a type alias for a function. What error will the TypeScript compiler report?

Typescript

type Operation = (x: number, y: number) => number;

const add: Operation = (x, y) => {
  return x + y;
};

const result: string = add(5, 3);

A Cannot assign to 'result' because it is a constant.

B Type 'number' is not assignable to type 'string'.

C Parameter 'y' implicitly has an 'any' type.

D No error, code compiles successfully.

💡 Hint

Look at the type of result and the return type of add.

The function add returns a number, but result is declared as a string. This causes a type mismatch error.

📝 Syntax

advanced

Which option correctly defines a type alias for a function with optional parameters?

Which of the following TypeScript type aliases correctly defines a function type that takes one required string parameter and one optional number parameter, returning void?

A type Func = (name: string, age?: number) => void;

B type Func = (name?: string, age: number) => void;

C type Func = (name: string, age: number?) => void;

D type Func = (name: string, age: number | undefined) => void;

💡 Hint

Optional parameters are marked with a question mark after the parameter name.

Option A correctly marks age as optional. Option A makes name optional instead. Option A has invalid syntax. Option A uses a union type but does not make the parameter optional.

🚀 Application

expert

How many items are in the resulting array after applying this function type alias?

Given the following TypeScript code, how many items will the results array contain after execution?

Typescript

type Transformer = (input: string) => string;

const transformers: Transformer[] = [
  (s) => s.toUpperCase(),
  (s) => s + "!",
  (s) => s.repeat(2)
];

const input = "hi";
const results = transformers.map(fn => fn(input));

A 3

B 1

C 0

D Error at runtime

💡 Hint

Count how many functions are in the transformers array.

There are 3 functions in the transformers array. The map method applies each function to the input string, so results will have 3 items.