Strict null checks and safety in Typescript

Introduction

Strict null checks help catch mistakes where a value might be missing or empty. This keeps your program safe and avoids crashes.

When you want to make sure a variable always has a real value before using it.

When you want to avoid errors caused by trying to use null or undefined values.

When you want TypeScript to help you find places where a value might be missing.

When working with user input that might be empty or missing.

When you want clearer and safer code that is easier to understand.

Syntax

Typescript

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

// Example variable declaration
let name: string | null = null;

// Checking before use
if (name !== null) {
  console.log(name.toUpperCase());
}

Setting strictNullChecks to true makes TypeScript check for null and undefined values strictly.

You must check if a value is null or undefined before using it, or TypeScript will show an error.

Examples

This example shows a number that can be null. We check before adding 1.

Typescript

let age: number | null = null;
age = 25;
if (age !== null) {
  console.log(age + 1);
}

This function greets a person if the name exists, otherwise it greets a guest.

Typescript

function greet(name: string | undefined) {
  if (name) {
    console.log(`Hello, ${name}!`);
  } else {
    console.log("Hello, guest!");
  }
}

Trying to use data.length without checking causes an error with strict null checks.

Typescript

let data: string | null = null;
// Error if strictNullChecks is true:
// console.log(data.length);

// Correct way:
if (data !== null) {
  console.log(data.length);
}

Sample Program

This program prints the length of a string if it exists, or a message if it is null.

Typescript

function printLength(text: string | null) {
  if (text === null) {
    console.log("No text provided.");
  } else {
    console.log(`Length: ${text.length}`);
  }
}

printLength(null);
printLength("Hello");

Important Notes

Always check for null or undefined before using values when strict null checks are on.

You can use ! (non-null assertion) to tell TypeScript a value is not null, but use it carefully.

Strict null checks make your code safer and easier to maintain.

Summary

Strict null checks help prevent errors by forcing you to handle null and undefined values.

Use checks like if (value !== null) before using values that can be null.

This feature makes your TypeScript code more reliable and clear.

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