JavaScript Simplified · One concept, one day, one project · Day 3 of 32

Data Types

The different kinds of value JavaScript can hold, and why the kind matters. You store a product’s details using the right type for each one.

Yesterday you stored a name (text) and an age (a number) without thinking about the difference. JavaScript does think about it. Every value has a type, and the type decides what you can do with the value.

1. Why Types Matter

Definition

A data type is the kind of value something is: text, a number, a yes/no, and so on. The type matters because JavaScript treats each kind differently. Adding two numbers gives a sum; "adding" two pieces of text joins them.

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// Numbers add up.
console.log(10 + 5);
// Text joins together (this is called concatenation).
console.log("10" + "5");

➜ webbo3 node app.js
15
105

2. The Basic Types

Type 1: String (text)

A string is text, written inside quotes. Single or double quotes both work, as long as they match.

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const firstName = "Ada";
const greeting = "Welcome back";
console.log(firstName);
console.log(greeting);

➜ webbo3 node app.js
Ada
Welcome back

Type 2: Number

A number is any number, written without quotes. Whole numbers and decimals are both just numbers in JavaScript.

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const age = 22;
const price = 1500.50;
console.log(age + 1);
console.log(price);

➜ webbo3 node app.js
23
1500.5

Type 3: Boolean (true or false)

A boolean is one of exactly two values: true or false. It answers a yes/no question, and it is the heart of every decision a program makes.

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const isLoggedIn = true;
const hasPaid = false;
console.log(isLoggedIn);
console.log(hasPaid);

➜ webbo3 node app.js
true
false

Type 4: Undefined and Null

undefined means "this variable has no value yet". JavaScript sets it automatically when you make a box but put nothing in it. null means "deliberately empty": you chose to put nothing here.

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let notSetYet;            // made, but nothing put in: undefined
let middleName = null;    // deliberately empty
console.log(notSetYet);
console.log(middleName);

➜ webbo3 node app.js
undefined
null

3. Checking a Type

When you are not sure what type a value is, typeof tells you. It is useful when a value came from somewhere else and you want to be certain.

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console.log(typeof "Ada");     // a string
console.log(typeof 22);        // a number
console.log(typeof true);      // a boolean
console.log(typeof undefined); // undefined

➜ webbo3 node app.js
string
number
boolean
undefined

Today’s Mini Project

📦 Product Information Card

Store a product’s details using the right type for each: a name (string), a price (number), whether it is in stock (boolean). Then print a card.

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const productName = "Wireless Mouse";   // string
const price = 7500;                      // number
const inStock = true;                    // boolean
const discount = null;                   // no discount set

console.log("Product: " + productName);
console.log("Price: N" + price);
console.log("In stock: " + inStock);
console.log("Discount: " + discount);

➜ webbo3 node app.js
Product: Wireless Mouse
Price: N7500
In stock: true
Discount: null

Checkpoint

Next: Day 4. Now that you have numbers and text, you will do things with them: add, compare, and combine, using operators.