Primitive data types in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are primitive data types in JavaScript?

Primitive data types are the most basic data types in JavaScript. They represent simple values and are not objects. Examples include string, number, boolean, null, undefined, symbol, and bigint.

beginner

What is the difference between null and undefined?

null means a variable has been explicitly set to have no value. undefined means a variable has been declared but not assigned any value yet.

beginner

Which primitive data type is used to represent true or false values?

The boolean data type represents logical values: true or false.

intermediate

What is a symbol in JavaScript?

A symbol is a unique and immutable primitive value often used to create unique object property keys to avoid name conflicts.

intermediate

How do you create a BigInt value in JavaScript?

You create a BigInt by appending n to the end of an integer literal, for example: 123n. BigInt is used for integers larger than the Number type can safely represent.

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

A Boolean

B String

C Object

D Symbol

Objects are not primitive data types; they are complex data types that can hold multiple values.

What value does an uninitialized variable hold in JavaScript?

A undefined

B false

C 0

D null

An uninitialized variable holds the value undefined.

Which primitive type would you use to store the text 'Hello'?

A String

B Number

C Boolean

D Symbol

Text values are stored as String primitive type.

How do you create a BigInt literal?

A By calling BigInt() without arguments

B By writing a number with a decimal point

C By using quotes around a number

D By appending 'n' to an integer

Appending 'n' to an integer creates a BigInt literal, e.g., 9007199254740991n.

Which primitive type is used to represent a unique identifier?

A Boolean

B Symbol

C Null

D Undefined

Symbol is used to create unique identifiers.

Explain the main primitive data types in JavaScript and give a simple example of each.

Think about the simplest values you can store in variables.

Describe the difference between null and undefined in JavaScript.

Consider what happens when you declare a variable but don’t assign it.

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