What are the data types in JavaScript and how are they classified?

There are the primitive types (string, number, boolean, null, undefined, symbol, bigint) and the object type (object), which includes arrays, functions, and plain objects. Primitives are copied by value; objects are copied by reference.

JavaScript has 7 primitive types:

• string: text, like "hola".
• number: numbers, both integers and decimals, like 42 or 3.14.
• boolean: true or false.
• undefined: a variable that was declared but has no assigned value.
• null: intentional absence of a value.
• symbol: unique identifiers (advanced use).
• bigint: very large integers.

Then there's object, which groups objects, arrays, and functions.

The key difference: primitives are copied by value (each variable gets its own copy), while objects are copied by reference (several variables can point to the same object).

console.log(typeof "hola");      // "string"
console.log(typeof 42);          // "number"
console.log(typeof true);        // "boolean"
console.log(typeof undefined);   // "undefined"
console.log(typeof null);        // "object"  (a historical bug in the language)
console.log(typeof Symbol());    // "symbol"
console.log(typeof 10n);         // "bigint"
console.log(typeof {});          // "object"
console.log(typeof []);          // "object"  (arrays are objects)
console.log(typeof function(){});// "function"

// Primitive: copied by value
let a = 1;
let b = a;
b = 2;
console.log(a); // 1 (a didn't change)

// Object: copied by reference
let obj1 = { x: 1 };
let obj2 = obj1;
obj2.x = 99;
console.log(obj1.x); // 99 (both point to the same object)

Saying that arrays are a separate type. In reality, typeof [] returns "object". To check whether something is an array, you use Array.isArray(valor). Another classic mistake: forgetting that typeof null is "object".

"JavaScript has seven primitive types: string, number, boolean, null, undefined, symbol, and bigint. Everything else is an object, including arrays and functions. The most important thing in practice is that primitives are copied by value and objects by reference, which affects how they behave when you assign them or pass them to functions."