Variables

Lecture 126:- Variables

In JavaScript, variables are used to store data values. They are like containers that hold values, which can be used later in the program. Variables can be declared using the var, let, or const keyword.

The var keyword was used to declare variables in JavaScript before the release of ES6. It has a function-level scope, which means that it can be accessed within the function it is declared in. If a variable is declared outside of a function using the var keyword, it will be accessible globally.

The let keyword was introduced in ES6 and has a block-level scope, which means that it can be accessed within the block it is declared in. A block is a set of statements enclosed in curly braces {}. Variables declared with let can be updated but not redeclared.

The const keyword is used to declare variables whose values are constant and cannot be changed. Like let, const has a block-level scope. Once a value is assigned to a variable using the const keyword, it cannot be reassigned.

Here's an example of declaring a variable using var, let, and const:

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// using var var x = 5; function myFunction() {  var y = 10;  console.log(x + y); // output: 15 } console.log(x); // output: 5 console.log(y); // output: Uncaught ReferenceError: y is not defined // using let let a = 15; if (true) {  let b = 10;  console.log(a + b); // output: 25 } console.log(a); // output: 15 console.log(b); // output: Uncaught ReferenceError: b is not defined // using const const PI = 3.14; console.log(PI); // output: 3.14 PI = 3.14159; // output: Uncaught TypeError: Assignment to constant variable.