Should I still use var?

In new code, no. let and const have block scope and avoid the hoisting surprises that var causes. You'll mainly see var in older code.

Why does typeof null return 'object'?

It's a historical bug from the first version of JavaScript that was never fixed to avoid breaking existing websites. Check for null explicitly with value === null.

JavaScript Variables and Data Types: let, const, and More

Introduction

Every JavaScript program starts with variables and values. This tutorial explains how to declare variables with const and let, the main data types, how to check a value's type, and why you should prefer strict equality.

Prerequisites

A modern web browser
Access to the browser developer console or Node.js

Step-by-Step Guide

Step 1: Declare variables with const and let

Use const for values that won't be reassigned and let for values that will. Prefer these over var, which has surprising function-level scope and hoisting behavior.

const siteName = "CaliberTek";
let visitors = 0;

visitors = visitors + 1;
console.log(siteName, visitors);   // CaliberTek 1

Step 2: Learn the primitive data types

JavaScript's primitives are string, number, boolean, undefined, null, bigint, and symbol. Use typeof to check a value's type.

console.log(typeof "hello");    // "string"
console.log(typeof 42);         // "number"
console.log(typeof true);       // "boolean"
console.log(typeof undefined);  // "undefined"
console.log(typeof null);       // "object" (a long-standing quirk)

Step 3: Compare values with strict equality

== converts types before comparing, which can hide bugs. === compares both value and type, so prefer it.

console.log(5 == "5");    // true  (type coercion)
console.log(5 === "5");   // false (different types)

Step 4: Work with objects and arrays

Objects store named properties and arrays store ordered lists. A const object or array can't be reassigned, but its contents can still change.

const user = { name: "Ava", skills: ["JavaScript", "CSS"] };

user.skills.push("HTML");
console.log(user.skills.length);   // 3

FAQs

Should I still use var?: In new code, no. let and const have block scope and avoid the hoisting surprises that var causes. You'll mainly see var in older code.
Why does typeof null return 'object'?: It's a historical bug from the first version of JavaScript that was never fixed to avoid breaking existing websites. Check for null explicitly with value === null.

Summary

Use const by default, let when you must reassign, know the primitive types, and compare with ===. These habits prevent many common beginner bugs.