Variables in JavaScript

JavaScript has three ways to make a variable. You need two of them.

const fee = 4500;     // will not be reassigned
let count = 0;        // will change
var old = "avoid";    // the old way

Use const by default

Start with const. Switch to let only when you actually reassign the name. Code where most names are const is easier to read, because a let becomes a signal that something changes.

const name = "Simran";
name = "Ravi";        // TypeError: Assignment to constant variable

const does not mean frozen

This is the part that confuses everybody. const stops the name being pointed somewhere else. It does not stop the value being changed:

const student = { name: "Simran" };
student.name = "Ravi";     // fine — same object, different contents
student.city = "Amritsar"; // also fine

student = {};              // TypeError — now you are moving the name

Same with arrays: const marks = [1,2]; marks.push(3); works. If you want the contents locked too, Object.freeze() does that, and you will rarely need it.

Why not var

var ignores blocks:

if (true) {
  var a = 1;
  let b = 2;
}
console.log(a);   // 1 — escaped the block
console.log(b);   // ReferenceError — stayed inside

var is also hoisted: you can use it before the line that declares it and get undefined instead of an error, which hides typos. You will still see var in older tutorials and in code you inherit. Read it, do not write it.

Types are decided by the value

let x = 5;          // number
x = "five";         // string — allowed
typeof x;           // 'string'

The types you will meet constantly: number, string, boolean, undefined, null, object (which includes arrays), and function.

One oddity to know before it bites: typeof null returns 'object'. It is a bug from 1995 that can never be fixed without breaking the web.