Strict null checks and safety in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output with strict null checks enabled

What is the output of this TypeScript code when strict null checks are enabled?

Typescript

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

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

A TypeError at runtime

B Hello, null!\nHello, ALICE!

C Hello, guest!\nHello, alice!

D Hello, guest!\nHello, ALICE!

💡 Hint

Check how the function handles the null case before calling toUpperCase.

The function checks if name is null and returns a special greeting. Otherwise, it safely calls toUpperCase on the string. This avoids runtime errors with null values.

❓ Predict Output

intermediate

Value of variable after null assertion

What is the value of variable 'length' after running this code with strict null checks enabled?

Typescript

let maybeString: string | null = "hello";
let length = maybeString!.length;

A 5

B undefined

C TypeError at runtime

D Compilation error

💡 Hint

The exclamation mark tells TypeScript the value is not null here.

The null assertion operator (!) tells TypeScript to treat maybeString as non-null. Since it actually holds "hello", length is 5.

❓ Predict Output

advanced

Output of function with optional chaining and strict null checks

What is the output of this TypeScript code with strict null checks enabled?

Typescript

type User = { name: string; address?: { city: string } };

function getCity(user: User): string {
  return user.address?.city ?? "Unknown city";
}

console.log(getCity({ name: "Bob", address: { city: "Paris" } }));
console.log(getCity({ name: "Alice" }));

A TypeError at runtime

B Paris\nundefined

C Paris\nUnknown city

D Compilation error

💡 Hint

Optional chaining safely accesses nested properties that might be missing.

The optional chaining operator (?.) returns undefined if address is missing, so the nullish coalescing operator (??) returns "Unknown city" instead.

❓ Predict Output

advanced

Result of filtering null values with strict null checks

What is the output of this TypeScript code with strict null checks enabled?

Typescript

const values: (string | null)[] = ["a", null, "b", null, "c"];
const filtered = values.filter(v => v !== null);
console.log(filtered);

A [null, null]

B ["a", "b", "c"]

C ["a", null, "b", null, "c"]

D Compilation error due to type mismatch

💡 Hint

The filter removes all null values from the array.

The filter callback keeps only elements not equal to null, so the resulting array contains only strings.

🧠 Conceptual

expert

Why does this code cause a compilation error with strict null checks?

Consider this TypeScript code with strict null checks enabled: let input: string | null = null; let length: number = input.length; Why does this cause a compilation error?

A Because input might be null, and accessing length on null is unsafe

B Because length is not a valid property of string

C Because null is not allowed as a value for string type

D Because TypeScript does not allow variables to be assigned before declaration

💡 Hint

Think about what happens if input is null when accessing a property.

With strict null checks, TypeScript prevents accessing properties on values that might be null to avoid runtime errors.

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