Type checking using typeof in Javascript - Time & Space Complexity

Time Complexity: Type checking using typeof

O(n)

Understanding Time Complexity

We want to understand how long it takes to check the type of a value using typeof in JavaScript.

Specifically, how does the time grow when we check many values?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const values = [1, 'hello', true, null, undefined, {}, []];
for (let i = 0; i < values.length; i++) {
  console.log(typeof values[i]);
}
    

This code loops through an array and prints the type of each element using typeof.

Identify Repeating Operations

• Primary operation: The for loop that runs typeof on each element.
• How many times: Once for each element in the array.

How Execution Grows With Input

As the number of elements grows, the number of typeof checks grows the same way.

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

Pattern observation: The work grows directly with the number of items; double the items, double the checks.

Final Time Complexity

Time Complexity: O(n)

This means the time to check types grows in a straight line with the number of elements.

Common Mistake

[X] Wrong: "Using typeof is instant and does not depend on the number of elements."

[OK] Correct: While one typeof is very fast, checking many elements means repeating it many times, so total time grows with the number of elements.

Interview Connect

Understanding how simple operations like typeof scale helps you reason about performance in real code, showing you can think about efficiency clearly.

Self-Check

"What if we replaced the for loop with a recursive function calling typeof on each element? How would the time complexity change?"

Practice

1. What does the typeof operator return when used on the value 42?

easy

A. "string"

B. "object"

C. "boolean"

D. "number"

Solution

1. Step 1: Understand the value type

   The value 42 is a number in JavaScript.

2. Step 2: Apply typeof operator

   Using typeof 42 returns the string "number" because 42 is a numeric value.

3. Final Answer:

   "number" -> Option D

4. Quick Check:

   typeof 42 = "number" [OK]

Hint: Remember: numbers return "number" with typeof [OK]

Common Mistakes:

• Thinking numbers return "string"
• Confusing number with object
• Using typeof without parentheses

2. Which of the following is the correct syntax to check if variable x is a string using typeof?

easy

A. if (typeof x = 'string') {}

B. if (typeof x === 'string') {}

C. if (typeof(x) == string) {}

D. if (typeof x === string) {}

Solution

1. Step 1: Check correct operator usage

   The strict equality operator is ===, so typeof x === 'string' is correct.

2. Step 2: Check string quotes and parentheses

   The type name must be a string literal in quotes, and parentheses around x are optional.

3. Final Answer:

   if (typeof x === 'string') {} -> Option B

4. Quick Check:

   Use === 'string' with quotes [OK]

Hint: Use triple equals and quotes around type name [OK]

Common Mistakes:

• Using single equals (=) instead of triple (===)
• Missing quotes around 'string'
• Using typeof without quotes for type

3. What will be the output of the following code?

console.log(typeof null);

medium

A. "null"

B. "undefined"

C. "object"

D. "boolean"

Solution

1. Step 1: Understand the value null

   In JavaScript, null is a special value representing "no object".

2. Step 2: Apply typeof to null

   Due to a historical bug, typeof null returns "object" instead of "null".

3. Final Answer:

   "object" -> Option C

4. Quick Check:

   typeof null = "object" [OK]

Hint: Remember: typeof null returns "object" (quirk) [OK]

Common Mistakes:

• Expecting "null" as output
• Confusing with "undefined"
• Thinking null is a boolean

4. Find the error in this code snippet:

let val = 10;
if (typeof val = "number") {
  console.log("It's a number");
}

medium

A. Using single equals (=) instead of comparison operator

B. Missing semicolon after let declaration

C. Incorrect quotes around "number"

D. typeof cannot be used on variables

Solution

1. Step 1: Check the if condition syntax

   The condition uses = which is assignment, not comparison.

2. Step 2: Correct operator for comparison

   Use === or == to compare the result of typeof val with "number".

3. Final Answer:

   Using single equals (=) instead of comparison operator -> Option A

4. Quick Check:

   Use === for comparison, not = [OK]

Hint: Use === for comparison, not = [OK]

Common Mistakes:

• Using = instead of === in if conditions
• Thinking typeof returns variable type directly
• Ignoring quotes around type string

5. You want to write a function isStringOrNumber that returns true if the input is either a string or a number, and false otherwise. Which of the following implementations is correct?

hard

A. function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; }

B. function isStringOrNumber(value) { return typeof value === 'string' && typeof value === 'number'; }

C. function isStringOrNumber(value) { return typeof value == 'string' && typeof value == 'number'; }

D. function isStringOrNumber(value) { return typeof value = 'string' || typeof value = 'number'; }

Solution

1. Step 1: Understand the requirement

   The function should return true if the value is a string OR a number.

2. Step 2: Check logical operators and comparisons

   function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; } uses || (OR) and strict equality ===, which is correct.

3. Step 3: Identify errors in other options

   function isStringOrNumber(value) { return typeof value === 'string' && typeof value === 'number'; } and the option using == uses AND &&, which requires both types simultaneously (impossible). function isStringOrNumber(value) { return typeof value = 'string' || typeof value = 'number'; } uses assignment = instead of comparison.

4. Final Answer:

   function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; } -> Option A

5. Quick Check:

   Use OR (||) and === for type checks [OK]

Hint: Use || for OR and === for type checks [OK]

Common Mistakes:

• Using && instead of || for OR condition
• Using = instead of === for comparison
• Mixing == and === inconsistently