Variable Declaration in JavaScript

1) In JavaScript, there are three keywords used for variable declaration: var, let, and const.

2) Variables created without a declaration keyword (var, let, or const) are always global, even if they are created inside a function. It becomes the property of the window object.

function example() {
  x = 10;
}
example();
console.log(window.x); // 10
console.log(x)         // 10

3) It's a good programming practice to declare all variables at the beginning of a script.

4) Objects (including arrays and functions) assigned to a variable using const are still mutable. Using the const declaration only prevents reassignment of the variable identifier.

const employee = { job:'Web Developer'};
employee.job = 'Data Scientist';
employee.name = 'Amey'
console.log(employee.job); // Data Scientist
console.log(employee); // { job: 'Data Scientist', name: 'Amey' }

const a = [1,2,3,4]
a[1]=7
a.push(100)
console.log(a) // [ 1, 7, 3, 4, 100 ]

5) Initialize const with declaration only otherwise it will give an error.

const x;
// Uncaught SyntaxError: Missing initializer in const declaration

const y = 10;

6) It is common for developers to use uppercase variable identifiers for immutable values and lowercase or camelCase for mutable values (objects and arrays).

const NAME = 'Jack';
let age = 30;

7) Limitations of Var keyword

Hoisting: Variables declared with var are hoisted, which means they are moved to the top of their scope, even before the code is executed. This can lead to unexpected behavior.

  console.log(x); 
    var x = 10;
    // Output: undefined

Function scope: In JavaScript, var variables are scoped to the nearest function, not block.

 function myFunction() {
     var x = 10;
     if (true) {
     var x = 20;
     }
    console.log(x); 
     }
    myFunction(); 
    // Output: 20

In this example, the var x inside the if block is not a new variable, but rather it's the same variable as the var x declared at the beginning of the function. Therefore, when you assign x the value of 20 inside the if block, it also changes the value of x in the entire function. The output of this code will be 20.
That's why use of let and const keywords is preferred over var.

Variable Declaration in JavaScript

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