Data Types in JavaScript

When you're learning JavaScript, one of the most important things to understand early on is data types. Every piece of data in your program—text, numbers, true/false values, or even empty values—has a specific type. Knowing how data is categorized helps you write better code and avoid bugs.

Primitive vs Reference Types

In JavaScript, data types fall into two groups: primitive types and reference types.

Primitive types are the basic building blocks. These are simple, single values like a name, a number, or a true/false choice. They are:

  • string
  • number
  • boolean
  • undefined
  • null
  • symbol
  • bigint

Each time you use a primitive value, it’s stored directly and separately in memory.

Reference types, on the other hand, are more complex. The main one is the object type. Objects group data together in a structured way, making it easier to manage related values.

Let’s break down each of the main data types you'll use all the time:

String

A string is text. You wrap it in quotes—single, double, or backticks.

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          let name = "Sara";
console.log(typeof name); // Output: "string"

Strings are used for things like names, messages, and anything written in words.

Number

A number is any whole or decimal value. It doesn’t need quotes.

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          let age = 25;
console.log(typeof age); // Output: "number"

let temperature = -3.5;
console.log(typeof temperature); // Output: "number"

Note If you put a number in quotes, JavaScript treats it as a string.

Boolean

Booleans are simple: they’re either true or false. They're great for making decisions.

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          let isOnline = true;
console.log(typeof isOnline); // Output: "boolean"

Undefined

This means a variable was created but hasn’t been given a value yet.

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          let city;
console.log(typeof city); // Output: "undefined"

Null

null is used when you intentionally want a variable to have no value.

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          let score = null;
console.log(typeof score); // Output: "object" (this is a known quirk)

Even though typeof null returns "object", it’s still considered a primitive value.

Symbol

Symbols are unique and can be used to create hidden or private object keys.

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          let id = Symbol("id");
console.log(typeof id); // Output: "symbol"

They’re mostly used in advanced programming but good to know they exist.

BigInt

Use this when you need really large numbers—bigger than JavaScript usually handles.

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          let big = 123456789123456789123456789n;
console.log(typeof big); // Output: "bigint"

Checking Types with typeof

You can find out what type any value is by using the typeof operator. It tells you whether something is a string, number, object, and so on.

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          console.log(typeof "Hello"); // Output: "string"
console.log(typeof 42);      // Output: "number"
console.log(typeof true);    // Output: "boolean"
console.log(typeof {});      // Output: "object"

You can use it with or without parentheses. Both work the same:

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          console.log(typeof(42)); // Output: "number"
console.log(typeof 42);  // Output: "number"

This tool is super helpful when you're debugging or just trying to understand your code better.