Typing

JS is dynamically typed

const a = 5; //a is a number
const b = 4.12; // b is a number 
const c = "some string"; //string
const d = [7,4,5,2]; // array

Types are adapted dynamically

let a = 5; //a is a number
a = "bear"; // now a is a string

const b = [4,"a string",[7,8]];

The same variable can contain various types at different part of the code
Good practice