7.

What are variables in JavaScript

Variables in JavaScript are named containers for storing data values. They allow programmers to label data with a descriptive name, so the data can be easily referenced and manipulated throughout the code. Variables in JavaScript can hold any type of data, such as numbers, strings, objects, functions, and more.

Declaring Variables

JavaScript provides three keywords for declaring variables: var, let, and const.

• var: Before ES6 (ECMAScript 2015), var was the only way to declare variables in JavaScript. Variables declared with var are function-scoped or globally-scoped if declared outside of a function. They are also hoisted, meaning they can be referenced in their scope even before they are declared.

  var name = 'Alice';

• let: Introduced in ES6, let allows developers to declare block-scoped variables. Unlike var, let does not hoist the variable to the top of the block.

  let age = 25;

• const: Also introduced in ES6, const is used to declare variables whose values are not intended to change. const declarations are block-scoped, similar to let. Attempting to reassign a const variable will result in a runtime error.

  const birthday = 'January 1, 1990';

Variable Naming Rules

When naming variables in JavaScript, there are a few rules and best practices to follow:

• Names can include letters, digits, underscores (_), and dollar signs ($).
• Names must begin with a letter, $, or _. They cannot start with a digit.
• JavaScript variable names are case-sensitive. For example, myVariable and myvariable are different variables.
• Reserved keywords (like if, return, function, etc.) cannot be used as variable names.
• It's recommended to use camelCase naming for variables in JavaScript.

Dynamic Typing

JavaScript is a dynamically typed language, meaning you don't need to specify the type of data a variable will hold. Instead, the type is determined at runtime, and the same variable can be reassigned to hold a different type of data.

let example = 'Hello, World!'; // example is a string
example = 42;                  // now example is a number
example = false;               // now example is a boolean

Scope

The scope of a variable determines where it can be accessed or modified in the code. As mentioned, var declarations are function-scoped or globally-scoped, while let and const declarations are block-scoped. Understanding scope is crucial for managing data effectively and avoiding unintended side effects in your code.

Use in Expressions

Variables can be used in expressions and operations, allowing for dynamic computation and manipulation of data.

let x = 10;
let y = 20;
let sum = x + y; // sum is now 30