Variables & Data Types

What you'll learn

Programs become useful when they can remember information. JavaScript stores values through named bindings and lets values carry their own types.

• Choose between const and let
• Recognize all primitive types and object values
• Inspect and convert values without accidental coercion
• Predict copying behavior for primitives and objects

const, let, and legacy var

A variable name is best pictured as a label attached to a value. Use const when the label will not be reassigned and let when it must point to a different value later.

const courseName = "JavaScript";
let completedLessons = 0;

completedLessons = completedLessons + 1;
console.log(courseName, completedLessons);

The seven primitive types

Primitives are immutable. A string method can produce a new string, but it cannot alter the original string value.

Reference values

Everything else is an object, including arrays and functions. Objects group mutable properties; variables hold references that lead to those objects.

const learner = { name: "Lina", level: 1 };
const topics = ["variables", "types"];

learner.level = 2;
topics.push("conversion");
console.log(learner.level, topics.length);

typeof and dynamic typing

JavaScript is dynamically typed: values have types, but a binding can later receive a value of another type. typeof reports a useful string for quick inspection.

let value = 42;
console.log(typeof value);     // number

value = "forty-two";
console.log(typeof value);     // string
console.log(typeof true);      // boolean
console.log(Array.isArray([])); // true

null vs undefined

undefined usually means JavaScript has no value yet: a declared-but-unassigned variable, a missing property, or a function with no return. null is normally written by a programmer to mean “intentionally empty.”

Explicit type conversion

Convert at boundaries—form input, URLs, storage, and APIs—so the rest of your code works with predictable types.

const input = "42";
const quantity = Number(input);

console.log(quantity + 8);
console.log(String(false));
console.log(Boolean(""));
console.log(Number.isNaN(Number("oops")));

Copying values vs references

let firstScore = 10;
let secondScore = firstScore;
secondScore = 99;

const firstUser = { name: "Sam" };
const secondUser = firstUser;
secondUser.name = "Mira";

console.log(firstScore, secondScore);
console.log(firstUser.name);

Primitive assignment copies the value. Object assignment copies the reference, so both variables reach the same object. A later lesson covers shallow and deep copying.

Recap & quick check