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Variables and Data Types in JavaScript

By Swann
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Published on
Variables and Data Types in JavaScript


Understanding variables and data types is foundational in embarking on the JavaScript programming journey. Variables are containers that hold data which can be manipulated during program execution, while data types define the nature and operations applicable to the data.

Variables in JavaScript

Declaring Variables

Variables in JavaScript are declared using var, let, or const keywords:

var name = "John";
let age = 30;
const pi = 3.14;

Scope and Hoisting

  • var declarations are function-scoped and are hoisted to the top of the function or global context.
  • let and const declarations are block-scoped and are not hoisted.

Mutable and Immutable Variables

  • Variables declared with var and let are mutable, meaning their values can be changed.
  • Variables declared with const are immutable in reference, meaning the reference cannot be changed.

Data Types in JavaScript

JavaScript supports various data types, which can be broadly categorized as primitive and object types.

Primitive Data Types

  • Number: Represents numerical values, e.g., let age = 25;
  • String: Represents a sequence of characters, e.g., let name = "Anna";
  • Boolean: Represents logical true or false, e.g., let isValid = false;
  • Undefined: Represents an uninitialized variable, e.g., let x;
  • Null: Represents an empty or non-existent value, e.g., let y = null;
  • Symbol: Represents a unique identifier, e.g., let sym = Symbol();
  • BigInt: Represents integers of arbitrary length, e.g., let bigNumber = 1234567890123456789012345678901234567890n;

Object Data Types

  • Object: A collection of properties, e.g., let person = {name: "John", age: 30};
  • Array: An ordered list of values, e.g., let fruits = ["apple", "banana", "cherry"];
  • Function: A block of code designed to perform a task, e.g., function greet() { console.log("Hello!"); }

Type Conversion and Coercion

JavaScript allows explicit type conversion and also performs type coercion when comparing values:

let str = String(123);  // Type conversion
let isTrue = '5' == 5;  // Type coercion (true)


A profound understanding of variables and data types in JavaScript is pivotal as they form the backbone for data storage and manipulation in scripting. This foundational knowledge aids in writing scripts that perform operations, make decisions, and manipulate DOM elements effectively on the web.

Additional Resources

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