Data types

Primitives

Every value in JavaScript has a type. There are seven primitive types, but you’ll use five of them daily: strings, numbers, booleans, null, and undefined.

// String - text wrapped in quotes
const name = "Sarah Doe";
const email = 'sarah@example.com';
const greeting = `Hello, ${name}!`; // Template literal (more on this next lesson)

// Number - integers and decimals are the same type
const age = 25;
const price = 99.99;
const temperature = -5;

// Boolean - true or false
const isLoggedIn = true;
const hasPermission = false;

// Null - intentionally empty ("I know there's nothing here")
const selectedItem = null;

// Undefined - not yet assigned ("This hasn't been set")
let userInput;
console.log(userInput); // undefined

That’s it. No int vs float, no str vs bytes. JavaScript keeps it simpler than Python in this regard.

JavaScript
Python

// One number type for everything
const count = 42;
const price = 19.99;
const negative = -10;

typeof count; // "number"
typeof price; // "number"

# Separate int and float types
count = 42
price = 19.99
negative = -10

type(count)  # <class 'int'>
type(price)  # <class 'float'>

null vs undefined

This is the one part that trips up Python developers. Python has None. JavaScript has two “nothing” values:

// null = you explicitly set it to "nothing"
let selectedUser = null; // "No user selected yet"

// undefined = JavaScript set it because nothing was assigned
let searchQuery;
console.log(searchQuery); // undefined - "This variable exists but has no value"

Use null when you want to intentionally say “empty” or “no value.” Let undefined be JavaScript’s default for “not yet assigned.” In practice, you’ll mostly use null.

Type checking

Use typeof to check what type a value is. This is especially useful when debugging or validating API responses.

typeof "Sarah";       // "string"
typeof 42;           // "number"
typeof true;         // "boolean"
typeof undefined;    // "undefined"
typeof null;         // "object"  ← This is a bug. Seriously.
typeof { name: "Sarah" }; // "object"
typeof [1, 2, 3];   // "object"  ← Arrays are objects too

typeof null returns "object" instead of "null". This is a well-known bug from 1995 that was never fixed because too much code depends on it. To check for null, use value === null instead.

// Checking for null properly
const selectedUser = null;

// ❌ Wrong: typeof doesn't help
console.log(typeof selectedUser); // "object"

// ✅ Correct: Direct comparison
console.log(selectedUser === null); // true

To check if something is an array, use Array.isArray():

const users = ["Sarah", "John", "Alice"];
const user = { name: "Sarah" };

Array.isArray(users); // true
Array.isArray(user);  // false

Type coercion

JavaScript tries to be “helpful” by automatically converting types when you mix them. This leads to some genuinely weird behavior.

// JavaScript automatically converts types
console.log("5" + 3);     // "53"  (number becomes string)
console.log("5" - 3);     // 2     (string becomes number)
console.log("5" * 2);     // 10    (string becomes number)
console.log(true + 1);    // 2     (true becomes 1)
console.log(false + 1);   // 1     (false becomes 0)
console.log("" == false); // true  (both coerced to 0)

The + operator is particularly tricky — if either side is a string, it concatenates instead of adding. The other math operators (-, *, /) convert strings to numbers.

Don’t rely on type coercion. Always convert types explicitly when you need to. Implicit coercion causes bugs that are hard to track down.

Converting types explicitly

// String to number
const input = "42";
const count = Number(input);
console.log(count);        // 42
console.log(typeof count); // "number"

// Number to string
const price = 99.99;
const label = String(price);
console.log(label);        // "99.99"
console.log(typeof label); // "string"

// Anything to boolean
Boolean(0);         // false
Boolean("");        // false
Boolean(null);      // false
Boolean(undefined); // false
Boolean("hello");   // true
Boolean(42);        // true
Boolean([]);        // true  ← Empty arrays are truthy!

Values that convert to false are called “falsy”: 0, "", null, undefined, NaN, and false. Everything else is “truthy.” You’ll use this concept constantly in React for conditional rendering.

Equality: == vs ===

This is the most important syntax rule in this entire lesson.

// == (loose equality) - converts types before comparing
console.log(5 == "5");    // true  (string "5" converted to number 5)
console.log(0 == false);  // true  (false converted to 0)
console.log("" == false); // true  (both converted to 0)
console.log(null == undefined); // true

// === (strict equality) - no type conversion, must match exactly
console.log(5 === "5");    // false (different types)
console.log(0 === false);  // false (different types)
console.log("" === false); // false (different types)
console.log(null === undefined); // false (different types)

JavaScript
Python

// Always use === in JavaScript
const userAge = "25";
const minAge = 25;

if (userAge === minAge) {
  console.log("Match"); // Won't run - different types
}

if (Number(userAge) === minAge) {
  console.log("Match"); // Runs - both are numbers now
}

# Python's == already works like JavaScript's ===
user_age = "25"
min_age = 25

if user_age == min_age:
    print("Match")  # Won't run - Python doesn't coerce

Always use === and !==. Pretend == and != don’t exist. This is the universal recommendation from every JavaScript style guide, linter, and senior developer.

Common mistakes

Using == instead of ===

// ❌ Wrong: Loose equality causes unexpected matches
const status = 0;
if (status == false) {
  console.log("No status"); // Runs! 0 is coerced to false
}

// ✅ Correct: Strict equality
const status = 0;
if (status === false) {
  console.log("No status"); // Doesn't run - 0 is not false
}
if (status === 0) {
  console.log("Status is zero"); // Runs - exact match
}

== converts types before comparing, which leads to surprising results like 0 == false being true. Use === to compare both value and type.

Confusing null and undefined

// ❌ Wrong: Treating them as interchangeable
let data;
if (data === null) {
  console.log("No data"); // Doesn't run! data is undefined, not null
}

// ✅ Correct: Check for both, or use the right one
let data;
if (data == null) {
  console.log("No data"); // Runs - == treats null and undefined as equal
}

// Or be explicit:
if (data === null || data === undefined) {
  console.log("No data"); // Runs
}

undefined means “not yet assigned.” null means “intentionally empty.” They’re not the same thing, but == treats them as equal. This is the one exception where == is sometimes preferred — checking for null == undefined catches both.

Assuming typeof null returns 'null'

// ❌ Wrong: Checking null with typeof
const result = null;
if (typeof result === "null") {
  console.log("Result is null"); // Never runs!
}

// ✅ Correct: Direct comparison
const result = null;
if (result === null) {
  console.log("Result is null"); // Runs
}

typeof null returns "object" due to a decades-old bug in JavaScript. Always check for null with === null.

What’s next?

You know the building blocks — strings, numbers, booleans, and how to compare them safely. Next, let’s look at a cleaner way to work with strings using template literals.

Template literals

String interpolation, multi-line strings, and goodbye to string concatenation

Fundamentals

Functions

Scope

Primitives

null vs undefined

Type checking

Type coercion

Converting types explicitly

Equality: == vs ===

Common mistakes

What’s next?

Template literals

​Primitives

​null vs undefined

​Type checking

​Type coercion

​Converting types explicitly

​Equality: == vs ===

​Common mistakes

​What’s next?

Template literals

Primitives

null vs undefined

Type checking

Type coercion

Converting types explicitly

Equality: == vs ===

Common mistakes

What’s next?