JavaScript Data Types

Every value in JavaScript has a type — a kind. A piece of text is a string, a count is a number, a yes/no answer is a boolean. The type decides what you can do with the value: you can add two numbers, but adding a number to a piece of text joins them instead. Knowing the types is knowing what your code is allowed to do.

Diagram — see how it fits together

There are eight types in total, but you will use six of them every day: string, number, boolean, undefined, null, and object. Arrays are objects with a special shape. The built-in operator typeof tells you the type of any value — it is the quickest way to check what you are really holding.

String, number, boolean — the three basics

A string is text inside quotes — single, double, or backtick quotes all work. A number is any count or measurement, whole or decimal; JavaScript has only one number type, so 10 and 10.5 are the same kind. A boolean is either true or false — the answer to a yes/no question, like "did the student pass?"

let name = "Rahul Verma";  // string
let marks = 82;            // number
let passed = true;         // boolean

undefined and null — two kinds of "nothing"

undefined means a variable exists but no value was ever put in it. null means a value was deliberately set to "nothing". The difference is intent: undefined is an accident waiting to be fixed, null is a choice — "this student has no middle name" is null, while a variable you forgot to fill is undefined. In practice, check for both when a value might be missing.

let middleName;           // undefined — never given a value
let nickname = null;      // null — deliberately empty

Object and array — values that hold other values

An object groups related values under named labels, called properties. A student has a name, an age, and a course — one object can hold all three. An array holds a list of values in order, reached by position: the first item is at position 0, the second at 1, and so on.

let student = { name: "Rahul", age: 19, course: "HTML" };
let subjects = ["HTML", "CSS", "JavaScript"];
student.name;    // "Rahul"
subjects[0];     // "HTML" — positions start at 0

typeof — asking a value what it is

The typeof operator returns the type as a word: typeof "Rahul" is "string", typeof 82 is "number", typeof true is "boolean". It has two famous quirks: typeof null says "object" (a mistake from the first version of the language, kept forever), and typeof [] also says "object" — arrays are objects. To check for an array properly, use Array.isArray().

Common mistakes

• Doing maths with a string from an input box — convert it with Number() first.
• Thinking typeof null should say "null" — it says "object", a historic quirk. Check null with === null instead.
• Using typeof to check for an array — it says "object". Use Array.isArray().
• Counting array positions from 1 — the first item is at position 0, so the last item of a 3-item list is at position 2.

Remember

• String = text in quotes, number = a count or measurement, boolean = true or false.
• undefined = never given a value; null = deliberately empty.
• An object groups values by name; an array lists values by position, starting at 0.
• typeof tells you a value's type — except for null and arrays, which both say "object".

Example explained

• let student = { name: "Rahul Verma", age: 19, course: "HTML" }; — one object holding three related facts about a student, each under its own label.
• let subjects = ["HTML", "CSS", "JavaScript"]; — an array of three strings, in the order the student is learning them.
• typeof student.name — asks the type of the name, and gets "string".
• typeof student.age — asks the type of the age, and gets "number".
• Array.isArray(subjects) — the proper way to check for an array, since typeof would only say "object".