Program structure

Expressions and statements

A fragment of code that produces a value is called an expression. Every value that is written literally (such as 22 or "psychoanalysis") is an expression. An expression between parentheses is also an expression, as is a binary operator applied to two expressions or a unary operator applied to one.

This shows part of the beauty of a language-based interface. Expressions can contain other expressions in a way similar to how subsentences in human languages are nested—a subsentence can contain its own subsentences, and so on. This allows us to build expressions that describe arbitrarily complex computations.

If an expression corresponds to a sentence fragment, a JavaScript statement corresponds to a full sentence. A program is a list of statements.

Bindings

How does a program keep an internal state? How does it remember things? We have seen how to produce new values from old values, but this does not change the old values, and the new value has to be immediately used or it will dissipate again. To catch and hold values, JavaScript provides a thing called a binding, or variable:

let caught = 5 * 5;

That’s a second kind of statement. The special word (keyword) let indicates that this sentence is going to define a binding. It is followed by the name of the binding and, if we want to immediately give it a value, by an = operator and an expression.

After a binding has been defined, its name can be used as an expression. The value of such an expression is the value the binding currently holds. Here’s an example:

let ten = 10;
console.log(ten * ten);
// → 100

When a binding points at a value, that does not mean it is tied to that value forever. The = operator can be used at any time on existing bindings to disconnect them from their current value and have them point to a new one.

let mood = "light";
console.log(mood);
// → light
mood = "dark";
console.log(mood);
// → dark

You should imagine bindings as tentacles, rather than boxes. They do not contain values; they grasp them—two bindings can refer to the same value. A program can access only the values that it still has a reference to. When you need to remember something, you grow a tentacle to hold on to it or you reattach one of your existing tentacles to it.

Luigis debt

Let’s look at another example. To remember the number of dollars that Luigi still owes you, you create a binding. And then when he pays back $35, you give this binding a new value.

let luigisDebt = 140;
luigisDebt = luigisDebt - 35;
console.log(luigisDebt);
// → 105