Type checking using typeof in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of typeof with different values

What is the output of the following code snippet?

Javascript

console.log(typeof null);

A "boolean"

B "object"

C "undefined"

D "null"

💡 Hint

Remember that typeof null is a known quirk in JavaScript.

In JavaScript, typeof null returns "object" due to legacy reasons.

❓ Predict Output

intermediate

Type of an array using typeof

What will be the output of this code?

Javascript

console.log(typeof [1, 2, 3]);

A "object"

B "array"

C "list"

D "undefined"

💡 Hint

Arrays are a special kind of object in JavaScript.

The typeof operator returns "object" for arrays because arrays are objects in JavaScript.

❓ Predict Output

advanced

Output of typeof with function and symbol

What is the output of this code snippet?

Javascript

console.log(typeof function() {});
console.log(typeof Symbol('id'));

A "function" and "symbol"

B "object" and "string"

C "function" and "object"

D "object" and "symbol"

💡 Hint

Functions have their own type, and symbols are a unique primitive type.

The typeof operator returns "function" for functions and "symbol" for symbols.

❓ Predict Output

advanced

Type of NaN using typeof

What will this code output?

Javascript

console.log(typeof NaN);

A "undefined"

B "NaN"

C "number"

D "object"

💡 Hint

NaN stands for Not-a-Number but is still a number type in JavaScript.

Despite its name, NaN is of type "number" in JavaScript.

❓ Predict Output

expert

Type checking with typeof and undefined variables

What is the output of this code?

Javascript

let a;
console.log(typeof a);
console.log(typeof b);

A ReferenceError and ReferenceError

B ReferenceError and "undefined"

C "undefined" and ReferenceError

D "undefined" and "undefined"

💡 Hint

typeof returns "undefined" for both declared but unassigned variables and undeclared variables without throwing errors.

Variable a is declared but not assigned, so typeof a returns "undefined". Variable b is undeclared, but typeof b also returns "undefined" safely without throwing a ReferenceError.

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