Coding Style::Javascript

Variable Names

camelCase for identifier names (variables and functions).
All names start with a letter.

firstName = "John";
lastName = "Doe";

price = 19.90;
tax = 0.20;

fullPrice = price + (price * tax);

Spaces Around Operators

Always put spaces around operators ( = + - * / ), and after commas:

var x = y + z;
var values = ["Volvo", "Saab", "Fiat"];

Code Indentation

Always use 4 spaces for indentation of code blocks:

function toCelsius(fahrenheit) {
    return (5 / 9) * (fahrenheit - 32);
}

Statement Rules

General rules for simple statements: Always end a simple statement with a semicolon.

var values = ["Volvo", "Saab", "Fiat"];

var person = {
    firstName: "John",
    lastName: "Doe",
    age: 50,
    eyeColor: "blue"
};

General rules for complex (compound) statements:

• Put the opening bracket at the end of the first line.
• Use one space before the opening bracket.
• Put the closing bracket on a new line, without leading spaces.
• Do not end a complex statement with a semicolon.

Functions:

function toCelsius(fahrenheit) {
    return (5 / 9) * (fahrenheit - 32);
}

Loops:

for (i = 0; i < 5; i++) {
    x += i;
}

Conditionals:

if (time < 20) {
    greeting = "Good day";
} else {
    greeting = "Good evening";
}

Object Rules

General rules for object definitions:

• Place the opening bracket on the same line as the object name.
• Use colon plus one space between each property and its value.
• Use quotes around string values, not around numeric values.
• Do not add a comma after the last property-value pair.
• Place the closing bracket on a new line, without leading spaces.
• Always end an object definition with a semicolon.

var person = {
    firstName: "John",
    lastName: "Doe",
    age: 50,
    eyeColor: "blue"
};

Short objects can be written compressed, on one line, using spaces only between properties, like this:

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

Line Length less than 80 never greater 120

For readability, avoid lines longer than 80 characters. Max 120 if you have to. If a JavaScript statement does not fit on one line, the best place to break it, is after an operator or a comma.

document.getElementById("demo").innerHTML =
    "Hello Dolly.";

Naming Conventions

Always use the same naming convention for all your code. For example:

• Variable and function names written as camelCase
• Global variables written in UPPERCASE (We don't, but it's quite common)
• Constants (like PI) written in UPPERCASE

Should you use hyp-hens, camelCase, or under_scores in variable names?

• Hyphens in HTML and CSS
• HTML5 attributes can start with data- (data-quantity, data-price).
• CSS uses hyphens in property-names (font-size).
• Hyphens can be mistaken as subtraction attempts.
• Hyphens are not allowed in JavaScript names.

Loading JavaScript in HTML

Use simple syntax for loading external scripts (the type attribute is not necessary):

<script src="myscript.js"></script>

Accessing HTML Elements

A consequence of using "untidy" HTML styles, might result in JavaScript errors.
These two JavaScript statements will produce different results:

var obj = getElementById("Demo")

var obj = getElementById("demo")

If possible, use the same naming convention (as JavaScript) in HTML.

File Extensions

HTML files should have a .html extension (not .htm).
CSS files should have a .css extension.
JavaScript files should have a .js extension.

Source: W3 Schools Javascript Coding Conventions