When and How Should I Use the `var` Keyword in JavaScript?

Understanding the Purpose and Usage of the 'var' Keyword in JavaScript

In JavaScript, the 'var' keyword plays a significant role in variable declaration, scope determination, and hoisting. However, when and how it should be used (or omitted) can be confusing. This article aims to clarify the purpose of 'var' and guide developers in making informed choices.

Function of the 'var' Keyword

Variable Declaration:
The primary purpose of 'var' is to declare variables in JavaScript. It allows developers to create named entities that can store values. For example:

var someNumber = 2;
var someFunction = function() { doSomething; }
var someObject = { };

Using 'var' ensures that these variables are created and initialized with the specified values.

Scope Determination

Global Scope:
When 'var' is used in the global scope, the variable it declares becomes globally accessible throughout the script. This behavior remains the same whether or not the 'var' keyword is present:

// Global variable creation
var someNumber = 2;
// Equivalent to
someNumber = 2;

Function Scope:
Within a function, using 'var' creates a local variable limited to the function's scope. However, if 'var' is omitted, JavaScript attempts to find the variable in the scope chain:

function()
{
    // Local variable creation
    var foo = 1;
    // Global variable access
    bar = 2;
}

In this example, 'foo' is a local variable, while 'bar' is a global variable.

Hoisting

Global Scope:
Variables declared with 'var' in the global scope are hoisted to the top of the script. This means they are effectively declared before any code is executed.

Function Scope:
Variables declared with 'var' within functions are also hoisted to the top of the function block but not the global scope.

When to Use 'var'

Required for Assignment:
When assigning a value to an already declared variable, 'var' is not necessary:

var x; // Variable declaration
x = 10; // Assignment

Conversely, if a variable is declared without 'var' and subsequently assigned, JavaScript creates a new global variable.

Variable Scope Control:
In cases where it's important to limit the scope of a variable, using 'var' within functions is crucial. This ensures the variable is only accessible within that function.

When to Omit 'var'

Global Scope Declaration:
When intentionally creating global variables, omitting 'var' can be a preferred choice, as JavaScript automatically assigns these variables to the global object.

Function-Scoped Variables with "let" (ECMAScript 6):
In newer versions of JavaScript (ECMAScript 6 ), the 'let' keyword can be used to declare block-scoped variables within functions, making 'var' less necessary for that purpose.

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