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strictNullChecks

Learn the option that enforces null safety.

What is strictNullChecks?

With strictNullChecks on, null and undefined become separate from other types. You cannot assign null to a string variable; you must write string | null for it to accept null. This prevents runtime null-read errors.

let t: string | null = null;
if (t !== null) {
  console.log(t.length);  // safe access
}

When strictNullChecks is off

With strictNullChecks off, null and undefined are assignable to any type. This flexibility is dangerous; if you access a property without checking for null, the app may crash.

let s: string = null;  // valid when off
console.log(s.length);  // runtime error

With it on, this line fails at compile time and you see the error early.

Practice Null-safe first element

Write a function firstOrNull(arr: string[]): string | null. If the array is empty, return null; otherwise return the first element. Print the result of both calls.

Starter code

index.ts
// firstOrNull(arr: string[]): string | null döndürür
// dizi boş ise null döndür
// değilse ilk elemanı döndür
// iki çağrıyı da yazdır

Sample solution

index.ts
function firstOrNull(arr: string[]): string | null {
  if (arr.length === 0) {
    return null;
  }
  return arr[0];
}

console.log(firstOrNull(["a", "b"]));
console.log(firstOrNull([]));

Expected output

a
null

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which assignment is valid when strictNullChecks is on?

  • let s: string = null;
  • let s: string; s = null;
  • let s: string = undefined;
  • let s: string | null = null; Correct answer

Explanation: With strictNullChecks on, you cannot assign null or undefined to a string variable. Writing string | null explicitly makes the null assignment valid.

2 Nullable return type

Question 
function getName(): string ____ { return null; }
Answer 
function getName(): string | null { return null; }

Explanation: With strictNullChecks on, you cannot return null from a string function. Writing string | null lets the function return null.

3 strictNullChecks parts

Term Meaning
strictNullChecks Makes null and undefined separate types
string | null A string that can be null
string Cannot be null under strict mode
t !== null Type narrowing

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app