Code

Let’s start from the beginning.

A program, often referred to as source code or just code, is a set of special instructions to tell the computer what tasks to perform. Usually code is saved in a text file, although with JavaScript you can also type code directly into a developer console in a browser, which we’ll cover shortly.

The rules for valid format and combinations of instructions is called a computer language, sometimes referred to as its syntax, much the same as English tells you how to spell words and how to create valid sentences using words and punctuation.

Statements

In a computer language, a group of words, numbers, and operators that performs a specific task is a statement. In JavaScript, a statement might look as follows:

a = b * 2;

The characters a and b are called variables (see “Variables”), which are like simple boxes you can store any of your stuff in. In programs, variables hold values (like the number 42) to be used by the program. Think of them as symbolic placeholders for the values themselves.

By contrast, the 2 is just a value itself, called a literal value, because it stands alone without being stored in a variable.

The = and * characters are operators (see “Operators”) — they perform actions with the values and variables such as assignment and mathematic multiplication.

Most statements in JavaScript conclude with a semicolon (;) at the end.

The statement a = b * 2; tells the computer, roughly, to get the current value stored in the variable b, multiply that value by 2, then store the result back into another variable we call a.

Programs are just collections of many such statements, which together describe all the steps that it takes to perform your program’s purpose.

Expressions

Statements are made up of one or more expressions. An expression is any reference to a variable or value, or a set of variable(s) and value(s) combined with operators.

For example:

a = b * 2;

This statement has four expressions in it:

• 2 is a literal value expression
• b is a variable expression, which means to retrieve its current value
• b * 2 is an arithmetic expression, which means to do the multiplication
• a = b * 2 is an assignment expression, which means to assign the result of the b * 2 expression to the variable a (more on assignments later)

A general expression that stands alone is also called an expression statement, such as the following:

b * 2;

This flavor of expression statement is not very common or useful, as generally it wouldn’t have any effect on the running of the program — it would retrieve the value of b and multiply it by 2, but then wouldn’t do anything with that result.

A more common expression statement is a call expression statement (see “Functions”), as the entire statement is the function call expression itself:

alert( a );

Executing a Program

How do those collections of programming statements tell the computer what to do? The program needs to be executed, also referred to as running the program.

Statements like a = b * 2 are helpful for developers when reading and writing, but are not actually in a form the computer can directly understand. So a special utility on the computer (either an interpreter or a compiler) is used to translate the code you write into commands a computer can understand.

For some computer languages, this translation of commands is typically done from top to bottom, line by line, every time the program is run, which is usually called interpreting the code.

For other languages, the translation is done ahead of time, called compiling the code, so when the program runs later, what’s running is actually the already compiled computer instructions ready to go.

It’s typically asserted that JavaScript is interpreted, because your JavaScript source code is processed each time it’s run. But that’s not entirely accurate. The JavaScript engine actually compiles the program on the fly and then immediately runs the compiled code.

Note: For more information on JavaScript compiling, see the first two chapters of the Scope & Closures title of this series.