Use const and let
Always declare variables with const or let, rather than var:
// Use let
let name = ‘John Doe’;
// Use const
const PI = 3.14;
var, let, and const are used to declare variables.
var is the oldest way to declare variables in JavaScript and has function scope. This means that a variable declared with var inside a function is only accessible within that function. If a var variable is declared outside of a function, it is considered a global variable and can be accessed from anywhere in the code.
var x = 10; // global scope
function example() {
var y = 20; // function scope
}
console.log(x); // 10
console.log(y); // ReferenceError: y is not defined
let and const are introduced in ES6 and are block-scoped, meaning that a variable declared with let or const is only accessible within the block it was declared in.
let x = 10; // global scope
if (true) {
let y = 20; // block scope
}
console.log(x); // 10
console.log(y); // ReferenceError: y is not defined
The difference between let and const is that let allows you to reassign the variable to a different value, while const does not. Once a variable is declared with const, its value cannot be changed.
const x = 10;
x = 20; // TypeError: Assignment to constant variable.
const y = [1, 2, 3];
y.push(4); // allowed, since arrays are mutable in JavaScript
It is recommended to use const by default, and only use let when you need to reassign the variable. This helps to ensure that your code is more readable, predictable, and secure.