JavaScript Syntax
Syntax Rules
Syntax are the rules how programs must be constructed:
// How to Declare variables:
let x = 5;
let y = 6;
// How to Compute values:
let z = x + y;
// I am a Comment. I do Nothing
JavaScript Values
The JavaScript syntax defines two types of values:
- Literals (Fixed values)
- Variables (Variable values)
JavaScript Literals
The most important syntax rules for literals (fixed values) are:
Numbers are written with or without decimals:
Strings are text, written within double or single quotes:
JavaScript Keywords
JavaScript keywords are used to defines actions to be performed.
The
let and const keywords create variables:
Note
JavaScript keywords are case-sensitive.
JavaScript does not interpret LET or Let as the keyword let.
JavaScript Variables
Variables are containers for storing data values.
Variables must be identified with unique names.
JavaScript Identifiers
An identifier is the name you give to a variable.
Rules for identifiers:
- Must start with a letter, _, or $
- Can contain digits after the first character
- Cannot be a reserved keyword (let, const, if, etc.)
- Are case-sensitive
JavaScript Operators
JavaScript assignment operators (=) assign values to variables:
JavaScript uses arithmetic operators ( + - * / ) to
compute values:
JavaScript Expressions
An expression is a combination of values, variables, and operators, which computes to a value.
Examples
(5 + 6) * 10 evaluates to 110:
(5 + 6) * 10
Try it Yourself »
Expressions can also contain variable:
x * 10
Try it Yourself »
"John" + " " + "Doe", evaluates to "John Doe":
"John" + " " + "Doe"
Try it Yourself »
JavaScript is Case Sensitive
JavaScript identifiers are case sensitive.
The variables lastName and lastname,
are different variables:
JavaScript and Camel Case
Historically, programmers have used different ways of joining multiple words into one variable name:
Hyphens:
first-name, last-name, master-card, inter-city.
Hyphens are not allowed in JavaScript. They are reserved for subtractions.
Underscore:
first_name, last_name, master_card, inter_city.
Upper Camel Case (Pascal Case):
FirstName, LastName, MasterCard, InterCity.
Lower Camel Case:
firstName, lastName, masterCard, interCity.
JavaScript programmers tend to use lower camel case.
