Primitive data types in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of typeof with different primitives

What is the output of the following code snippet?

Javascript

console.log(typeof null);
console.log(typeof undefined);
console.log(typeof 42);
console.log(typeof 'hello');

A

"object"
"undefined"
"number"
"string"

B

"null"
"undefined"
"number"
"string"

C

"object"
"null"
"number"
"string"

D

"undefined"
"undefined"
"number"
"string"

💡 Hint

Remember that typeof null is a known JavaScript quirk.

In JavaScript, typeof null returns "object" due to legacy reasons. typeof undefined returns "undefined". Numbers and strings return "number" and "string" respectively.

❓ Predict Output

intermediate

Boolean conversion of different values

What is the output of this code?

Javascript

console.log(Boolean(0));
console.log(Boolean(''));
console.log(Boolean('false'));
console.log(Boolean(null));

A

false
false
false
false

B

true
false
true
false

C

false
true
true
false

D

false
false
true
false

💡 Hint

Check which values are considered falsy in JavaScript.

In JavaScript, 0, empty string '', and null are falsy values, so Boolean conversion returns false. The string 'false' is a non-empty string, so it is truthy and converts to true.

🧠 Conceptual

advanced

Understanding Symbol uniqueness

Which statement about JavaScript Symbols is true?

A Symbols can be used as unique object property keys.

B Symbols are primitive but can be converted to strings automatically.

C Two Symbols created with Symbol() are always equal.

D Symbols are mutable primitive values.

💡 Hint

Think about what makes Symbols special compared to strings.

Symbols are unique and immutable primitive values. Each call to Symbol() creates a new unique symbol, so two symbols are never equal. Symbols can be used as unique keys for object properties. They cannot be automatically converted to strings; explicit conversion is needed.

❓ Predict Output

advanced

Output of typeof with BigInt and other primitives

What is the output of this code?

Javascript

console.log(typeof 9007199254740991n);
console.log(typeof true);
console.log(typeof '123');
console.log(typeof undefined);

A

"number"
"boolean"
"string"
"undefined"

B

"bigint"
"boolean"
"string"
"undefined"

C

"bigint"
"boolean"
"number"
"undefined"

D

"bigint"
"boolean"
"string"
"null"

💡 Hint

BigInt is a newer primitive type in JavaScript.

The typeof operator returns "bigint" for BigInt values, "boolean" for booleans, "string" for strings, and "undefined" for undefined. null is not returned by typeof undefined.

🔧 Debug

expert

Identify the error in primitive assignment

What error will this code produce?

Javascript

const str = 'hello';
str[0] = 'H';
console.log(str);

A TypeError: Cannot assign to read only property '0' of string

B SyntaxError: Invalid assignment

C hello

D undefined

💡 Hint

Strings are immutable in JavaScript. What happens when you try to change a character?

Strings are immutable primitives in JavaScript. Trying to assign to an index like str[0] = 'H' does not change the string and does not throw an error. The original string remains unchanged, so console.log(str) outputs hello.

Practice

1. Which of the following is NOT a primitive data type in JavaScript?

easy

A. object

B. boolean

C. string

D. number

Solution

1. Step 1: Recall JavaScript primitive types

   JavaScript primitive types include string, number, boolean, undefined, null, symbol, and bigint.

2. Step 2: Identify non-primitive types

   Object is not a primitive type; it is a complex data type used to store collections.

3. Final Answer:

   object -> Option A

4. Quick Check:

   Primitive types exclude objects [OK]

Hint: Remember: objects are NOT primitives, others are [OK]

Common Mistakes:

• Confusing object with primitive types
• Thinking null is an object
• Mixing symbol with object

2. Which of the following is the correct way to declare a variable with a string primitive in JavaScript?

easy

A. let name = John;

B. let name = 'John';

C. let name = "John;

D. let name = `John;

Solution

1. Step 1: Check string declaration syntax

   Strings must be enclosed in matching quotes: single ('') or double ("") or backticks (``).

2. Step 2: Validate each option

   let name = 'John'; uses single quotes correctly. let name = John; lacks quotes. Options C and D have unclosed quotes.

3. Final Answer:

   let name = 'John'; -> Option B

4. Quick Check:

   Strings need matching quotes [OK]

Hint: Strings always need matching quotes around text [OK]

Common Mistakes:

• Missing quotes around string
• Using mismatched quotes
• Forgetting semicolon (optional but good)

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

console.log(typeof 123);
console.log(typeof 'hello');
console.log(typeof true);

medium

A. "number", "string", "boolean"

B. "Number", "String", "Boolean"

C. "num", "str", "bool"

D. "integer", "text", "bool"

Solution

1. Step 1: Understand typeof operator

   typeof returns the type of a value as a lowercase string like "number", "string", or "boolean".

2. Step 2: Check each console.log output

   typeof 123 is "number", typeof 'hello' is "string", typeof true is "boolean".

3. Final Answer:

   "number", "string", "boolean" -> Option A

4. Quick Check:

   typeof returns lowercase type names [OK]

Hint: typeof returns lowercase type names as strings [OK]

Common Mistakes:

• Capitalizing type names
• Using short forms like 'num'
• Confusing typeof output with values

4. Find the error in this code snippet:

let value = null;
console.log(typeof value);

medium

A. null is not a valid value

B. typeof null returns 'null'

C. typeof null returns 'object' which is confusing

D. No error, code works fine

Solution

1. Step 1: Understand typeof null behavior

   typeof null returns "object" due to a historical JavaScript bug.

2. Step 2: Identify confusion

   This is confusing because null is a primitive but typeof shows "object".

3. Final Answer:

   typeof null returns 'object' which is confusing -> Option C

4. Quick Check:

   typeof null is 'object' (quirk) [OK]

Hint: Remember typeof null is 'object' due to JavaScript quirk [OK]

Common Mistakes:

• Expecting 'null' from typeof
• Thinking null is an object
• Assuming code throws error

5. You want to store a very large integer number beyond the safe integer limit in JavaScript. Which primitive type should you use?

hard

A. number

B. symbol

C. string

D. bigint

Solution

1. Step 1: Understand number limits in JavaScript

   The number type can safely represent integers only up to 2^53 - 1.

2. Step 2: Identify type for large integers

   bigint is designed to store integers larger than the safe limit without losing precision.

3. Final Answer:

   bigint -> Option D

4. Quick Check:

   Use bigint for very large integers [OK]

Hint: Use bigint for integers larger than safe number limit [OK]

Common Mistakes:

• Using number for very large integers
• Using string instead of bigint
• Confusing symbol with number types