How TypeScript infers types automatically - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the inferred type of variable 'x'?

Consider this TypeScript code snippet. What is the inferred type of variable x?

Typescript

let x = 42;

A number

B any

C string

D unknown

💡 Hint

TypeScript looks at the value assigned to infer the type.

Since 42 is a number, TypeScript infers the type of x as number.

❓ Predict Output

intermediate

What is the inferred type of the function parameter?

Look at this function. What type does TypeScript infer for the parameter name?

Typescript

function greet(name = "Guest") {
  return `Hello, ${name}!`;
}

A any

B unknown

C string

D number

💡 Hint

Default value helps TypeScript guess the parameter type.

The default value "Guest" is a string, so TypeScript infers the parameter type as string.

❓ Predict Output

advanced

What is the inferred type of the array elements?

What type does TypeScript infer for the elements of this array?

Typescript

const fruits = ["apple", "banana", "cherry"];

A string[]

B any[]

C Array<number>

D unknown[]

💡 Hint

All elements are strings, so TypeScript infers an array of strings.

Since all elements are strings, TypeScript infers the type as string array (string[]).

❓ Predict Output

advanced

What is the inferred return type of this arrow function?

What type does TypeScript infer as the return type of this arrow function?

Typescript

const multiply = (a: number, b: number) => a * b;

A void

B number

C any

D string

💡 Hint

The function returns the product of two numbers.

The function returns the product of two numbers, so the return type is inferred as number.

❓ Predict Output

expert

What is the inferred type of the variable 'result'?

Given this code, what is the inferred type of result?

Typescript

const data = [1, "two", 3];
const result = data.map(item => typeof item === "number" ? item * 2 : item.toUpperCase());

A any[]

B number[]

C string[]

D (string | number)[]

💡 Hint

The map returns either a number or a string depending on the item type.

The map function returns numbers doubled or strings uppercased, so the result is an array of strings or numbers.