Anatomy of a JavaScript program

A JavaScript program is essentially a list of instructions for the computer to follow to in order to achieve whatever useful thing the program is required to do.

Statements

Each instruction in a program is called a statement

In JavaScript, statements are composed of values, operators, expressions and keywords, and end with a semicolon.

The following is a statement:

const foo = 3 + 4;

In the above statement a variable value foo is created using the const keyword, and assigned a value using the assignment operator. This value is determined by evaluating the expression 3 + 4, which combines the literal values 3 and 4 using the addition operator.

Most programs contain lots and lots of statements, which get executed consecutively, in the order they are written down.

Values

There are actually two types of values in JavaScript, literal values, and variable values.

Literal values

A literal value is exactly what it sounds like - a value that is written down in it's literal, raw form.

In JavaScript this raw data comes in 5 main forms:

• Numbers
• Strings (aka text)
• Booleans
• Arrays
• Objects

Numbers, strings and booleans are known as primitives
Arrays and objects are known as data structures

Some examples of literal values:

6;
'Hello';
true;
[1, 2, 3];
{ name: 'Fred', age: 79 };

Variable values

A variable is like a container used to store a literal value or a function. It provides a reference to a value, and can give a more semantic meaning to a value, so that code becomes both easier to read, easier to write and less repetitive.

JavaScript uses the keywords var, const and let to declare (create) variables, and the = operator to assign values to variables.

var, const and let are each slightly different. The primary difference is semantic rather than functional:

• let represents a variable that will be reassigned and change it's value;
• const represents a constant value that will not change.
• var pre-dates the introduction of const and let, and doesn't have any semantic connotations. It's use is still quite common, but is not recommended in modern JavaScript.

The following code declares a variable foo, and assigns it the literal value 'bar' (a string).

let foo;
foo = 'bar';

Keywords

Keywords are basically reserved words that perform special actions and cannot be used as variable names.
For example, the keyword const creates a new variable.
Trying to create a variable called let is a not valid JavaScript syntax.

const let = 10;

Operators

Operators are special symbols that are used to combine two values and create a new one.

The main types of operator are:

• Arithmetic operators - used on numbers (e.g. * for multiplication)
• String operators - used on strings (e.g. + for concatination)
• Logical operators - used on booleans (e.g. ||, the OR operator)
• Comparison operators - used assess a relationship between values (e.g. ===, the equality operator)
• Assignment operators - used to set or alter the value of a variable (e.g. +=, the addition-assignemnt operator)

Expressions

An expression is a combination of literal values, variables, and operators, which evaluates to a single value.

1 + 2;

The above code is an expression that evaluates to 3

Blocks

JavaScript statements can be grouped together in blocks using curly braces ({}).

This is useful because it allows you to define statements that should be executed together.

Blocks are normally found in functions, if-statements and loops