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JavaScript syntax is the set of rules, how JavaScript programs are constructed
The JavaScript syntax defines two types of values:
Fixed values are called Literals.
Variable values are called Variables.
The two most important syntax rules for fixed values are:
1. Numbers are written with or without decimals:
10.50
1001
2. Strings are text, written within double or single quotes:
"John Doe"
'John Doe'
In a programming language, variables are used to store data values.
JavaScript uses the keywords var
,
let
and const
to declare variables.
An equal sign is used to assign values to variables.
In this example, x is defined as a variable. Then, x is assigned (given) the value 6:
let x;
x = 6;
JavaScript uses arithmetic operators ( +
-
*
/
) to
compute values:
(5 + 6) * 10
JavaScript uses an assignment operator ( =
) to assign
values to variables:
let x, y;
x = 5;
y = 6;
An expression is a combination of values, variables, and operators, which computes to a value.
The computation is called an evaluation.
For example, 5 * 10 evaluates to 50:
5 * 10
Expressions can also contain variable values:
x * 10
The values can be of various types, such as numbers and strings.
For example, "John" + " " + "Doe", evaluates to "John Doe":
"John" + " " + "Doe"
JavaScript keywords are used to identify actions to be performed.
The
let
keyword tells the browser to create variables:
let x, y;
x = 5 + 6;
y = x * 10;
The
var
keyword also tells the browser to create variables:
var x, y;
x = 5 + 6;
y = x * 10;
In these examples, using var
or let
will produce the same result.
You will learn more about var
and let
later in this tutorial.
Not all JavaScript statements are "executed".
Code after double slashes
//
or between /*
and */
is treated as a comment.
Comments are ignored, and will not be executed:
let x = 5; // I will be executed
// x = 6; I will
NOT be executed
You will learn more about comments in a later chapter.
Identifiers are JavaScript names.
Identifiers are used to name variables and keywords, and functions.
The rules for legal names are the same in most programming languages.
A JavaScript name must begin with:
Subsequent characters may be letters, digits, underscores, or dollar signs.
Numbers are not allowed as the first character in names.
This way JavaScript can easily distinguish identifiers from numbers.
All JavaScript identifiers are case sensitive.
The variables lastName
and lastname
,
are two different variables:
let lastname, lastName;
lastName = "Doe";
lastname = "Peterson";
JavaScript does not interpret LET or Let as the keyword let.
Historically, programmers have used different ways of joining multiple words into one variable name:
first-name, last-name, master-card, inter-city.
Hyphens are not allowed in JavaScript. They are reserved for subtractions.
first_name, last_name, master_card, inter_city.
FirstName, LastName, MasterCard, InterCity.
JavaScript programmers tend to use camel case that starts with a lowercase letter:
firstName, lastName, masterCard, interCity.
JavaScript uses the Unicode character set.
Unicode covers (almost) all the characters, punctuations, and symbols in the world.
For a closer look, please study our Complete Unicode Reference.