Type checking using typeof in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the typeof operator do in JavaScript?

It tells you the type of a value or variable, like whether it is a number, string, boolean, object, function, or undefined.

beginner

What is the result of typeof 42?

The result is "number" because 42 is a number.

intermediate

What does typeof null return and why is it special?

It returns "object". This is a known quirk in JavaScript where null is considered an object type even though it means 'no value'.

beginner

How can you check if a variable is a function using typeof?

Use typeof variable === 'function'. This returns true if the variable holds a function.

intermediate

What type does typeof return for arrays?

It returns "object" because arrays are a special kind of object in JavaScript.

What will typeof 'hello' return?

A "string"

B "text"

C "char"

D "object"

typeof returns "string" for text values.

Which typeof result means the variable is not defined or has no value?

A "null"

B "undefined"

C "void"

D "none"

"undefined" means a variable has no assigned value.

What does typeof function() {} return?

A "method"

B "object"

C "function"

D "callable"

Functions return "function" when checked with typeof.

What is the typeof result for an array like []?

A "object"

B "collection"

C "list"

D "array"

Arrays are objects in JavaScript, so typeof returns "object".

Why is typeof null considered a quirk?

A Because it returns "undefined"

B Because it throws an error

C Because it returns "null"

D Because it returns "object"

typeof null returns "object" even though null means no value.

Explain how the typeof operator helps in checking variable types in JavaScript.

Think about how you can tell what kind of value a variable holds.

Describe the special case of typeof null and why it might confuse beginners.

Consider what null means and what typeof returns.

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