Null and undefined types in Typescript - Time & Space Complexity

Time Complexity: Null and undefined types

O(n)

Understanding Time Complexity

We want to understand how checking for null or undefined values affects the speed of a program.

Specifically, how does the time to run grow when we check many values for null or undefined?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function countValid(values: (string | null | undefined)[]): number {
  let count = 0;
  for (const val of values) {
    if (val !== null && val !== undefined) {
      count++;
    }
  }
  return count;
}
    

This code counts how many items in an array are not null or undefined.

Identify Repeating Operations

• Primary operation: Looping through each item in the array once.
• How many times: Exactly once for every item in the input array.

How Execution Grows With Input

As the number of items grows, the time to check each item grows the same way.

Input Size (n)	Approx. Operations
10	About 10 checks
100	About 100 checks
1000	About 1000 checks

Pattern observation: The number of checks grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line as the input list gets bigger.

Common Mistake

[X] Wrong: "Checking for null or undefined is slow and adds extra loops."

[OK] Correct: The check happens inside the same loop, so it does not add extra loops or multiply the time.

Interview Connect

Understanding how simple checks inside loops affect performance helps you write clear and efficient code.

Self-Check

"What if we nested another loop inside to check each character of each string? How would the time complexity change?"

Practice

1. What does null represent in TypeScript?

easy

A. A variable that has not been declared

B. A variable that holds a number zero

C. A variable that is still loading

D. A variable that has no value assigned

Solution

1. Step 1: Understand the meaning of null

   null means a variable explicitly has no value.

2. Step 2: Compare with other options

   The incorrect options describe different states or values, not null.

3. Final Answer:

   A variable that has no value assigned -> Option D

4. Quick Check:

   null means no value [OK]

Hint: Remember: null means no value assigned [OK]

Common Mistakes:

• Confusing null with undefined
• Thinking null means variable is undeclared
• Assuming null means zero or empty string

2. Which of the following is the correct way to declare a variable that can hold a string or null in TypeScript?

easy

A. let name: string | null;

B. let name: string & null;

C. let name: string or null;

D. let name: string? null;

Solution

1. Step 1: Recall union type syntax

   In TypeScript, the pipe | is used to allow multiple types.

2. Step 2: Check each option

   let name: string | null; uses | correctly. The other options use invalid syntax.

3. Final Answer:

   let name: string | null; -> Option A

4. Quick Check:

   Union types use | symbol [OK]

Hint: Use | to combine types like string | null [OK]

Common Mistakes:

• Using & instead of | for union types
• Writing 'or' instead of |
• Using invalid syntax like ? null

3. What will be the output of this TypeScript code?

let value: number | undefined;
console.log(value);

medium

A. undefined

B. null

C. 0

D. Error: Variable not initialized

Solution

1. Step 1: Understand variable initialization

   The variable value is declared but not assigned, so it defaults to undefined.

2. Step 2: Check console.log output

   Logging value prints undefined because it has no assigned value.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   Unassigned variable = undefined [OK]

Hint: Uninitialized variables print undefined [OK]

Common Mistakes:

• Expecting null instead of undefined
• Assuming default is zero
• Thinking it causes an error

4. Identify the error in this TypeScript code:

let age: number;
age = null;

medium

A. No error, code is correct

B. Variable age is not declared

C. Cannot assign null to a variable of type number

D. Missing semicolon after declaration

Solution

1. Step 1: Check variable type and assignment

   age is declared as number only, so it cannot hold null.

2. Step 2: Understand TypeScript strict null checks

   By default, null is not assignable to number unless union type includes null.

3. Final Answer:

   Cannot assign null to a variable of type number -> Option C

4. Quick Check:

   Assigning null to number causes error [OK]

Hint: Add | null to allow null assignment [OK]

Common Mistakes:

• Ignoring TypeScript strict null checks
• Thinking null is allowed by default
• Confusing missing semicolon as error

5. How can you declare a function parameter in TypeScript that accepts either a string, null, or undefined?

hard

A. function greet(name?: string & null) { console.log(name); }

B. function greet(name: string | null | undefined) { console.log(name); }

C. function greet(name: string & null & undefined) { console.log(name); }

D. function greet(name: string | null?) { console.log(name); }

Solution

1. Step 1: Understand union types for parameters

   To accept multiple types, use | to combine them.

2. Step 2: Analyze each option

   function greet(name: string | null | undefined) { console.log(name); } correctly uses string | null | undefined. function greet(name: string & null & undefined) { console.log(name); } uses & which means intersection (not correct). function greet(name?: string & null) { console.log(name); } uses & which means intersection (not correct). function greet(name: string | null?) { console.log(name); } has invalid syntax.

3. Final Answer:

   function greet(name: string | null | undefined) { console.log(name); } -> Option B

4. Quick Check:

   Use | to allow multiple types in parameters [OK]

Hint: Use | to combine string, null, and undefined types [OK]

Common Mistakes:

• Using & instead of | for union types
• Confusing optional parameter with undefined union
• Writing invalid syntax like null?