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Data Types

Q: What does it mean that JavaScript is 'dynamically typed'?

Answer: It means that a variable is not tied to a specific data type. You can assign a number to a variable and later assign a string to the same variable without causing an error..

Q: What is the difference between 'undefined' and 'null'?

Answer: 'undefined' means a variable has been declared but has not yet been assigned a value. 'null' is an assignment value that represents 'no value' or a null pointer..

Q: What is returned by 'typeof' when checked on an array?

Answer: The 'typeof' operator returns 'object' for arrays, as arrays are a specialized type of object in JavaScript..

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Expert Answer & Key Takeaways

Mastering Data Types is essential for high-fidelity technical architecture and senior engineering roles in 2026.

JavaScript Data Types

JavaScript variables can hold many types of data: numbers, strings, objects and more. JavaScript is a dynamically typed language, meaning the same variable can be used to hold different data types.

1. The Concept of Data Types

In programming, data types are an important concept. To be able to operate on variables, it is important to know something about the type.

let x = 16 + "Volvo"; // Result: "16Volvo"

When adding a number and a string, JavaScript will treat the number as a string.

2. Primitive Data Types

JavaScript has 7 primitive data types (plus null):

String: Text content inside quotes. "Hello JS".
Number: Used for all types of numbers (integer, decimal). 3.14.
BigInt: Large integers beyond the Number limit. 9007199254740991n.
Boolean: Represents a logical entity with two values: true or false.
Undefined: A variable that has not been assigned a value has the type undefined.
Null: Represents the intentional absence of any object value.
Symbol: A unique and immutable primitive value.

3. Complex Data Types

The most important complex data type is the Object.

Objects

JavaScript objects are written with curly braces {}. Object properties are written as name:value pairs, separated by commas.

const person = {firstName:"John", lastName:"Doe", age:50};

Arrays

JavaScript arrays are written with square brackets []. Array items are separated by commas.

const cars = ["Saab", "Volvo", "BMW"];

4. The `typeof` Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns a string representing the type.

typeof "John";              // Returns "string"
typeof 3.14;                // Returns "number"
typeof {name:'John'};       // Returns "object"
typeof [1,2,3];             // Returns "object" (Arrays are objects!)

[!IMPORTANT] In JavaScript, an array is technically a type of object. typeof [1,2,3] will return "object".

Data Types

Expert Answer & Key Takeaways

JavaScript Data Types

1. The Concept of Data Types

2. Primitive Data Types

3. Complex Data Types

Objects

Arrays

4. The `typeof` Operator

Top Interview Questions

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Course4All Engineering Team

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Data Types

Expert Answer & Key Takeaways

JavaScript Data Types

1. The Concept of Data Types

2. Primitive Data Types

3. Complex Data Types

Objects

Arrays

4. The typeof Operator

Top Interview Questions

?Interview Question

?Interview Question

?Interview Question

Course4All Engineering Team

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Strings & Templates

Numbers & BigInt

Booleans & Logic

Operators

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4. The `typeof` Operator