In Javascript, variables are used to store and manipulate data.
Variables are containers for values and are declared using keywords like let, const, or var.
Variable names can include letters, numbers, underscores `_`, and dollar signs `$`.
"Variable names" cannot start with a number.
Javascript is case-sensitive, so yourVariableName and yourvariablename will treated as different variables.
The `let` variable was introduced in ES6 (ECMAScript 2015) to address some issues with the `var` variable.
`let` variables have blocked-scope. They can be accessible within the same block of code.
Use the `let` keyword to declare a variable that can be reassigned.
The `let` variable can't be re-declared in the same block scope.
let name = 'Alice'; let age = 20;
The `const` variable was introduced in ES6 (ECMAScript 2015) to address some issues with the `var` variable.
`const` variables have blocked-scope. They can be accessible within the same block of code.
Use const to declare a `constant` variable whose value cannot be reassigned:
The `const` variables can't be re-declared in the same block scope.
function constBlockScope() {
const isStudent = true;
// with `const` re-declare or reassign variable won't work
// const isStudent = false;
if (isStudent) {
const isTopper = false;
console.log(isStudent); // Output: true
}
// through error becoz const has block-scoped
// console.log(isTopper); // Output: false
}
constBlockScope();
The var keyword was originally used to create variables in Javascript before the introduction of `let` and `const`, but now it is considered outdated. It has some differences in scoping behaviour compared to let and const.
`var` variables have functional-scoped.
If we declare a `var` variable outside of any function, the variable becomes a global variable, which can lead to unintended consequences and potential issues with variable hoisting.
function varFunctionalScope() {
var isStudent = true;
// with `var` keyword we can re-declare same variable in function or block scope
// var isStudent = true;
if (isStudent) {
var isTopper = false;
console.log(isStudent); // Output: true
}
console.log(isTopper); // Output: false
// if we re-declare `var` variable in code it won't through error.
isStudent = false
}
varFunctionalScope();
// if we un-comment this line we will encounter an error
// console.log(isStudent); // Error: `isStudent` is not defined
Note: `var` variables are visible throughout the function in which they are declared.