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.

Practice

1.

What does enabling strictNullChecks in TypeScript do?

easy

A. It allows assigning null to any type without errors.

B. It automatically converts all null values to empty strings.

C. It disables type checking for null and undefined.

D. It forces you to explicitly handle null and undefined values.

Solution

1. Step 1: Understand strictNullChecks purpose

   Enabling strictNullChecks makes TypeScript require explicit handling of null and undefined values to avoid runtime errors.

2. Step 2: Analyze options

   Only It forces you to explicitly handle null and undefined values. correctly describes this behavior. Options B, C, and D are incorrect because they describe behaviors that strictNullChecks does not do.

3. Final Answer:

   It forces you to explicitly handle null and undefined values. -> Option D

4. Quick Check:

   strictNullChecks = explicit null handling [OK]

Hint: Remember: strictNullChecks means no silent nulls [OK]

Common Mistakes:

• Thinking nulls are automatically converted
• Assuming null checks are disabled
• Believing null can be assigned anywhere

2.

Which of the following is the correct way to check for null or undefined before accessing a variable value in TypeScript with strictNullChecks enabled?

let value: string | null | undefined;

easy

A. if (value !== null && value !== undefined) { console.log(value.length); }

B. if (value) { console.log(value.length); }

C. console.log(value.length);

D. if (value == null) { console.log(value.length); }

Solution

1. Step 1: Understand null and undefined checks

   With strictNullChecks, you must check both null and undefined explicitly before accessing properties.

2. Step 2: Evaluate each option

   if (value !== null && value !== undefined) { console.log(value.length); } correctly checks both null and undefined. if (value) { console.log(value.length); } may fail if value is an empty string. console.log(value.length); causes an error. if (value == null) { console.log(value.length); } checks for null but then accesses length inside the if, which is incorrect logic.

3. Final Answer:

   if (value !== null && value !== undefined) { console.log(value.length); } -> Option A

4. Quick Check:

   Check both null and undefined explicitly [OK]

Hint: Use !== null && !== undefined to be safe [OK]

Common Mistakes:

• Using only if(value) which fails on empty strings
• Accessing properties without checks
• Using == null but then accessing inside that block

3.

What will be the output of the following TypeScript code with strictNullChecks enabled?

function greet(name: string | null) {
  if (name === null) {
    return "Hello, guest!";
  }
  return `Hello, ${name.toUpperCase()}!`;
}

console.log(greet(null));
console.log(greet("alice"));

medium

A. "Hello, null!" and "Hello, ALICE!"

B. "Hello, guest!" and "Hello, ALICE!"

C. Compilation error due to missing null check

D. Runtime error on null input

Solution

1. Step 1: Analyze null check in function

   The function checks if name === null and returns "Hello, guest!" in that case, so it safely handles null.

2. Step 2: Check outputs for calls

   Calling greet(null) returns "Hello, guest!". Calling greet("alice") returns "Hello, ALICE!" because toUpperCase() is called on a string.

3. Final Answer:

   "Hello, guest!" and "Hello, ALICE!" -> Option B

4. Quick Check:

   Null check prevents errors, outputs as expected [OK]

Hint: Check null before using string methods [OK]

Common Mistakes:

• Expecting runtime error on null
• Thinking compilation error occurs
• Ignoring null check and causing error

4.

Identify the error in this TypeScript code with strictNullChecks enabled and suggest a fix:

function printLength(text: string | null) {
  console.log(text.length);
}

medium

A. Error: Object is possibly 'null'. Fix: Add a null check before accessing length.

B. Error: Missing return type. Fix: Add : void to function.

C. Error: text should be string only. Fix: Remove null from type.

D. No error. Code is correct.

Solution

1. Step 1: Identify the error

   Accessing text.length without checking if text is null causes a compile error under strictNullChecks.

2. Step 2: Suggest fix

   Add a check like if (text !== null) before accessing length to ensure safety.

3. Final Answer:

   Error: Object is possibly 'null'. Fix: Add a null check before accessing length. -> Option A

4. Quick Check:

   Always check null before property access [OK]

Hint: Add null checks before property access [OK]

Common Mistakes:

• Ignoring null check and expecting no error
• Changing type instead of fixing code
• Thinking return type causes error

5.

Given this TypeScript function with strictNullChecks enabled, which modification safely handles undefined and null inputs without errors?

function formatName(name: string | null | undefined) {
  return name.toUpperCase();
}

hard

A. Use non-null assertion: return name!.toUpperCase();

B. Change parameter type to string only and remove null and undefined.

C. Add a check: if (name == null) return "Unknown"; before calling toUpperCase().

D. Remove strictNullChecks from tsconfig.

Solution

1. Step 1: Identify unsafe code

   Calling toUpperCase() on name without checking for null or undefined causes errors with strictNullChecks.

2. Step 2: Evaluate options for safety

   Add a check: if (name == null) return "Unknown"; before calling toUpperCase(). adds a check for both null and undefined (using == null) and returns a default string, safely avoiding errors. Change parameter type to string only and remove null and undefined. removes null safety but is less flexible. Use non-null assertion: return name!.toUpperCase(); forces non-null assertion which can cause runtime errors. Remove strictNullChecks from tsconfig. disables safety feature, not recommended.

3. Final Answer:

   Add a check: if (name == null) return "Unknown"; before calling toUpperCase(). -> Option C

4. Quick Check:

   Check null/undefined before method calls [OK]

Hint: Use if(name == null) to check null and undefined together [OK]

Common Mistakes:

• Using non-null assertion unsafely
• Removing null from type instead of checking
• Disabling strictNullChecks instead of fixing code