Primitive Values

Primitive Values​ are numbers and strings, among other things. Open your
browser’s console and print these primitive values using console.log():

console.log(2);
console.log(“hello”);
console.log(undefined);

All primitive values have something in common. ​There’s nothing I can do in my
code that would affect them.​ This sounds a bit vague, so we’ll explore what
this means concretely in the next module. For now, I’ll say that primitive values
are like stars — cold and distant, but always there when I need them.

That’s the first kind of values.

Objects and Functions
Objects and Functions​ are also values, but they are not primitive. This makes
them very special. Go ahead and log a few of them to the browser console:

console.log({});
console.log([]);
console.log(x => x * 2);

Objects and functions are special because ​I can manipulate them from my
code​. For example, I can connect them to other values. This is rather vague —
so we’ll refine this idea in a later module. For now, I can say that if primitive
values are like distant stars, then objects and functions are more like rocks
floating nearby my code. They’re close enough that I can manipulate them.

And that’s the second kind of values.

You might have questions. Good. If you ask a question, the JavaScript universe
might answer it! Provided, of course, that you know how to ask.

Expressions
There are many questions JavaScript can’t answer. If you want to know
whether it’s better to confess your true feelings to your best friend or to keep
waiting until you both turn into skeletons, JavaScript won’t be of much help.

But there are some questions that JavaScript would be ​delighted​ to answer.
These questions have a special name — they are called ​expressions​.

If we “ask” the expression 2 + 2, JavaScript will “answer” with the value 4.

console.log(2 + 2); // 4

Expressions are questions that JavaScript can answer. JavaScript answers
expressions in the only way it knows how — with values.

console.log(2+2

If the word “expression” confuses you, think of it as a piece of code that
expresses​ a value. You might hear people say that 2 + 2 “results in” or
“evaluates to” 4. These are all different ways to say the same thing.

We ask JavaScript 2 + 2, and it answers with 4. ​Expressions always result in a
single value.​ Now we know enough about expressions to be dangerous!

I previously said that there are many types of JavaScript values: numbers,
strings, objects, and so on. How do we know any particular value’s type?

This sounds like a question. Do we dare to ask it?

Checking a Type
At first, all values in the JavaScript cosmos might look the same — bright dots
in the sky. But if you look closely, you’ll realize there are fewer than ten different
types of values. Values of the same type behave in similar ways.

If we want to check a value’s type, we can ask it with the typeof operator.
JavaScript will answer our question with one of the predetermined string
values, such as “number”, “string”, or “object”.

Below are a few examples you can try in the browser console:

console.log(typeof(2)); // “number”
console.log(typeof(“hello”)); // “string”
console.log(typeof(undefined)); // “undefined”

Here, typeof(2) is an expression — and it results in the “number” value.

Strictly saying, using parens isn’t required with typeof. For example, typeof 2
would work just as fine as typeof(2). However, sometimes parens are required
to avoid an ambiguity. One of the cases below would break if we omitted the
parens after typeof. Try to guess which one it is:

console.log(typeof({})); // “object”
console.log(typeof([])); // “object”
console.log(typeof(x => x * 2)); // “function”

We can verify your guess in the browser console.

Now take another look at the last three examples — this time with close
attention to their results. Did you find any of these results surprising? Why?

Types of Values
After almost twenty five years of
studying JavaScript, the scientists have only discovered nine such types:

Primitive Values

● Undefined​ (undefined), used for unintentionally missing values.
● Null​ (null), used for intentionally missing values.
● Booleans​ (true and false), used for logical operations.
● Numbers​ (-100, 3.14, and others), used for math calculations.
● Strings​ (“hello”, “abracadabra”, and others), used for text.
● Symbols​ (uncommon), used to hide implementation details.
● BigInts​ (uncommon and new), used for math on big numbers.

Objects and Functions

● Objects​ ({} and others), used to group related data and code.
● Functions​ (x => x * 2 and others), used to refer to code.

No Other Types
You might ask: “But what about other types I have used, like arrays?”

In JavaScript, there are no other fundamental value types other than the ones
we have just enumerated.​ The rest are all objects! For example, even arrays,
dates, and regular expressions fundamentally ​are​ objects in JavaScript:

console.log(typeof([])); // “object”
console.log(typeof(new Date())); // “object”
console.log(typeof(/(hello|goodbye)/)); //
“object”

“I see,” ​everything​ is an object!” Alas, this is a
popular urban legend, but it’s not true. Although code like “hi”.toUpperCase()
makes “hi” seem like an object, this is nothing but an illusion. JavaScript
creates a wrapper object when you do this, and then immediately discards it.

It’s fine if this mechanism doesn’t quite click yet. ​For now, you only need to
remember that primitive values, such as numbers and strings, are ​not​ objects.

Recap
Let’s recap what we know so far:

1. There are values, and then there’s everything else.​ We can think of
values as different things “floating” in our JavaScript universe. They
don’t exist ​inside​ our code, but we can refer to them from our code.
2. There are two categories of values: there are ​Primitive Values​, and
then there are ​Objects and Functions​.​ In total, there are nine separate
types. Each type serves a specific purpose, but some are rarely used.
3. Some values are lonely.​ For example, null is the only value of the Null
type, and undefined is the only value of the Undefined type. As we will
learn later, these two lonely values are quite the troublemakers!
4. We can ask questions with expressions.​ JavaScript will answer to us
with values. For example, the 2 + 2 expression is answered with 4.
5. We can inspect the type of something by wrapping it in a typeof
expression.​ For example, typeof(4) is the string value “number”.