JavaScript Style Guide
Always use the same coding conventions for all your JavaScript projects.
JavaScript Coding Conventions
Coding conventions are style guidelines for programming. They typically cover:
- Naming and declaration rules for variables and functions.
- Rules for the use of white space, indentation, and comments.
- Programming practices and principles
Coding conventions secure quality:
- Improves code readability
- Make code maintenance easier
Coding conventions can be documented rules for teams to follow, or just be your individual coding practice.
This page describes the general JavaScript code conventions used by W3Schools.
You should also read the next chapter "Best Practices", and learn how to avoid coding pitfalls.
Variable Names
At W3schools we use camelCase for identifier names (variables and functions).
All names start with a letter.
At the bottom of this page, you will find a wider discussion about naming rules.
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:
Examples:
let x = y + z;
const myArray = ["Volvo", "Saab", "Fiat"];
Code Indentation
Always use 2 spaces for indentation of code blocks:
Functions:
function toCelsius(fahrenheit) {
return (5 / 9) * (fahrenheit - 32);
}
Do not use tabs (tabulators) for indentation. Different editors interpret tabs differently.
Statement Rules
General rules for simple statements:
- Always end a simple statement with a semicolon.
Examples:
const cars = ["Volvo", "Saab", "Fiat"];
const 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 (let i = 0; i < 5; i++) {
x += i;
}
Conditionals:
if (time < 20) {
greeting = "Good day";
} 别的 {
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.
例子
const 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:
const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Line Length < 80
For readability, avoid lines longer than 80 characters.
If a JavaScript statement does not fit on one line, the best place to break it, is after an operator or a comma.
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?
This is a question programmers often discuss. The answer depends on whom you ask:
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.
Underscores:
Many programmers prefer to use underscores (date_of_birth), especially in SQL databases.
Underscores are often used in PHP documentation.
PascalCase:
PascalCase is often preferred by C programmers.
camelCase:
camelCase is used by JavaScript itself, by jQuery, and other JavaScript libraries.
Do not start names with a $ sign. It will put you in conflict with many JavaScript library 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:
const obj = getElementById("Demo")
const obj = getElementById("demo")
If possible, use the same naming convention (as JavaScript) in HTML.
File Extensions
HTML files should have a .html extension (.htm is allowed).
CSS files should have a .css extension.
JavaScript files should have a .js extension.
Use Lower Case File Names
Most web servers (Apache, Unix) are case-sensitive about file names:
london.jpg cannot be accessed as London.jpg.
Other web servers (Microsoft, IIS) are not case-sensitive:
london.jpg can be accessed as London.jpg or london.jpg.
If you use a mix of upper and lower case, you have to be extremely consistent.
If you move from a case-insensitive, to a case-sensitive server, even small errors can break your website.
To avoid these problems, always use lower case file names (if possible).
Performance
Coding conventions are not used by computers. Most rules have little impact on the execution of programs.
Indentation and extra spaces are not significant in small scripts.
For code in development, readability should be preferred. Larger production scripts should be minified.
