The most fundamental concept in JavaScript is the value. The JavaScript universe contains several values, and when I write code, I refer to these values and interact with them. However, they do not exist inside my code.

There are nine distinct types of values in JavaScript and they are grouped into two categories, with values of the same type behaving similarly: primitive values and objects and functions.

Primitive Values

Primitive values are the building blocks of JavaScript:

• String, used to represent text
• Number, used for math calculations
• BigInt represent numeric values which are too large to be represented by the Number primitive type
• Boolean, used for logical operations (true and false)
• undefined, used for unintentionally missing values
• null, used for intentionally missing values
• Symbol, used to create unique identifiers

I can't create, destroy, or modify primitive values. I can only point to them. To see the primitive values in action, open the browser console and log them:

console.log(42);
console.log("hi");
console.log(undefined);

Objects and Functions

Objects and functions are values that are not part of my code, but that I can manipulate with my code.

• Objects ({}), used to group related data and code together
• Functions ((x, y) => x + y), used to refer to code

To see objects and functions in action, open the browser console and log them:

console.log({});
console.log([]);
console.log((x, y) => x + y);

Checking a Type

The value types listed above are all that there is in JavaScript. There are no other types. Anything else is an object.

But what if I have a value and I want to know what type it is? I can ask JavaScript a question and it will answer with a single value. This question that you can ask JavaScript is called an expression, which is a fancy way of saying that it is a piece of code that expresses a value.

It is important to note that expressions are not values, but actually exist in our code.

In fact, if I wanted to know the type of a value, I could ask JavaScript a question with the typeof operator. JavaScript will answer with a string value indicating the type of that particular value:

console.log(typeof 42); // "number"
console.log(typeof "hi"); // "string"
console.log(typeof undefined); // "undefined"

console.log(typeof []); // "object"
console.log(typeof new Date()); // "object"
console.log(typeof /(hi|there)/); // "object"

console.log(typeof null); // "object" -> this is a bug in JavaScript
console.log(typeof typeof value); // "string"