Strict null checks and safety in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable that cannot be null or undefined.

Typescript

let name: string = [1];

Drag options to blanks, or click blank then click option'

A"Alice"

Bnull

Cundefined

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning null or undefined to a variable typed as string without union type.

With strict null checks, a variable of type string cannot be assigned null or undefined. Assigning a string literal like "Alice" is correct.

2fill in blank

medium

Complete the code to allow the variable to hold either a string or null.

Typescript

let username: string[1] = null;

Drag options to blanks, or click blank then click option'

A& null

B| null

C? null

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of | for union types.

Using ? incorrectly for union types.

Using | null creates a union type allowing username to be a string or null.

3fill in blank

hard

Fix the error by completing the code to safely access the length of a possibly null string.

Typescript

function getLength(str: string | null): number | undefined {
  return str[1].length;
}

Drag options to blanks, or click blank then click option'

A??

B!

C.

D?.

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot operator directly causes error if str is null.

Using non-null assertion operator (!) does not check for null.

The optional chaining operator ?. safely accesses length only if str is not null.

4fill in blank

hard

Fill both blanks to check if a variable is not null before accessing its property.

Typescript

if (user [1] null [2] user.name) {
  console.log(user.name);
}

Drag options to blanks, or click blank then click option'

A!==

B&&

C==

D||

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of !== for null check.

Using || instead of && causing incorrect logic.

We check that user is not null with !== and then use && to ensure user.name exists before logging.

5fill in blank

hard

Fill all three blanks to create a function that returns the length of a string or zero if null or undefined.

Typescript

function safeLength(str: string[1]): number {
  return str?.length[2][3];
}

Drag options to blanks, or click blank then click option'

A| null | undefined

B??

C0

D||

Attempts:

3 left

💡 Hint

Common Mistakes

Using || instead of ?? which treats empty string as false.

Not allowing undefined in the type.

The parameter type allows string, null, or undefined. The nullish coalescing operator ?? returns 0 if str is null or undefined.