Null and undefined types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with null and undefined

What is the output of this TypeScript code?

Typescript

function checkValue(x: number | null | undefined) {
  if (x === null) return "Null";
  if (x === undefined) return "Undefined";
  return x * 2;
}

console.log(checkValue(null));
console.log(checkValue(undefined));
console.log(checkValue(5));

A "Null"\n"Undefined"\n10

B null\nundefined\n10

C "Undefined"\n"Null"\n10

D TypeError at runtime

💡 Hint

Check how the function compares the input to null and undefined explicitly.

The function returns "Null" if the input is exactly null, "Undefined" if exactly undefined, otherwise doubles the number. So the outputs are "Null", "Undefined", and 10.

❓ Predict Output

intermediate

Value of variable after assignment

What is the value of variable result after running this code?

Typescript

let value: string | null | undefined = undefined;
value = value ?? "default";
const result = value;

A undefined

B "default"

C null

D TypeError

💡 Hint

The nullish coalescing operator (??) returns the right side if the left side is null or undefined.

Since value is initially undefined, value ?? "default" evaluates to "default". So result is "default".

🔧 Debug

advanced

Why does this code cause an error?

This TypeScript code causes a compile-time error. What is the reason?

Typescript

function greet(name: string | null) {
  console.log("Hello, " + name.toUpperCase());
}

greet(null);

A toUpperCase() is not a function in TypeScript

B Function greet must return a string

C "name" can be null, so calling toUpperCase() causes an error

D null is not assignable to parameter of type string

💡 Hint

Think about what happens if name is null and you call a method on it.

The parameter name can be null, but the code calls name.toUpperCase() without checking. This causes a compile-time error because null has no methods.

🧠 Conceptual

advanced

Difference between null and undefined in TypeScript

Which statement correctly describes the difference between null and undefined in TypeScript?

A <code>null</code> means a variable has no value; <code>undefined</code> means a variable has not been assigned a value

B <code>undefined</code> means a variable has no value; <code>null</code> means a variable has not been assigned a value

C <code>null</code> and <code>undefined</code> are exactly the same in TypeScript

D <code>null</code> is a type; <code>undefined</code> is a value

💡 Hint

Think about what happens when you declare a variable but don't assign it, versus assigning null explicitly.

In TypeScript, undefined means a variable has not been assigned a value yet, while null is an explicit assignment meaning 'no value'.

❓ Predict Output

expert

Output of complex null and undefined checks

What is the output of this TypeScript code?

Typescript

function test(x: string | null | undefined) {
  switch (x) {
    case null:
      return "Null case";
    case undefined:
      return "Undefined case";
    default:
      return x.length;
  }
}

console.log(test(null));
console.log(test(undefined));
console.log(test("hello"));

A "Null case"\n"Undefined case"\n0

B "Undefined case"\n"Null case"\n5

C TypeError at runtime

D "Null case"\n"Undefined case"\n5

💡 Hint

Remember how switch compares values and how string length works.

The switch matches null and undefined cases exactly, returning strings. For "hello", it returns length 5.