Generic function syntax in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a generic identity function

What is the output of this TypeScript code when calling identity(42)?

Typescript

function identity<T>(arg: T): T {
  return arg;
}

console.log(identity(42));

A undefined

B "42"

C 42

D TypeError

💡 Hint

The function returns exactly what it receives.

The generic function identity returns the argument it receives without changes. Passing 42 returns 42.

❓ Predict Output

intermediate

Output of generic function with array argument

What will this code print to the console?

Typescript

function firstElement<T>(arr: T[]): T | undefined {
  return arr[0];
}

console.log(firstElement(['apple', 'banana', 'cherry']));

A "banana"

B "apple"

C undefined

D TypeError

💡 Hint

The function returns the first item of the array.

The generic function returns the first element of the array passed. The first element is "apple".

❓ Predict Output

advanced

Output of generic function with multiple type parameters

What is the output of this code?

Typescript

function pairToString<K, V>(key: K, value: V): string {
  return `${key}: ${value}`;
}

console.log(pairToString<number, string>(1, "one"));

A "1: one"

B "one: 1"

C "[object Object]: one"

D TypeError

💡 Hint

The function formats key and value as a string separated by a colon.

The function returns a string combining the key and value separated by ': '. The key is 1 and value is "one".

❓ Predict Output

advanced

Output of generic function with constraints

What will this code output?

Typescript

interface Lengthwise {
  length: number;
}

function loggingLength<T extends Lengthwise>(arg: T): number {
  return arg.length;
}

console.log(loggingLength('hello'));

A 5

B "hello"

C TypeError

D undefined

💡 Hint

Strings have a length property.

The generic function requires the argument to have a length property. The string "hello" has length 5.

🧠 Conceptual

expert

Type inference in generic functions

Given the generic function function wrap(value: T): { value: T }, what is the inferred type of T when calling wrap([1, 2, 3])?

A unknown

B number

C any[]

D number[]

💡 Hint

The type parameter matches the argument's type exactly.

The argument is an array of numbers, so TypeScript infers T as number[].