JavaScript Style Guide¶
This document defines JavaScript style conventions, which are split into essential, coding and naming conventions.
Essential Conventions¶
Essential conventions include generic patterns that you should adhere to in order to write readable, consistent and maintainable code.
Minimizing Globals¶
Variable declarations should always be done using var to not declare them as global variables. This avoids conflicts from using a variable name across different functions as well as conflicts with global variables declared by third party plugins.
Good Example
function sum(x, y) {
var result = x + y;
return result;
}
Bad Example
function sum(x, y) {
// missing var declaration, implied global
result = x + y;
return result;
}
Using JSLint¶
JSLint is a quality tool that inspects code and warns about potential problems. It can be used online and can also be integrated into several development environments, so errors can be highlighted while writing code.
Before validating your code in JSLint, you should use a code beautifier to fix basic syntax errors (like indentation) automatically. There are a number of beautifiers available online. The following ones seem to work best:
In this project, JavaScript sources have to begin with the header:
/*jslint indent: 2, maxlen: 80, nomen: true */
which means it uses two spaces indentation, 80 maximum characters per line and allows variable names starting with ‘_’. Other JSLint options can be added in sub functions if necessary.
Some allowed options are:
ass: true
if assignment should be allowed outside of statement position.bitwise: true
if bitwise operators should be allowed.continue: true
if the continue statement should be allowed.newcap: true
if Initial Caps with constructor function is optional.regexp: true
if.
and[^...]
should be allowed in RegExp literals. They match more material than might be expected, allowing attackers to confuse applications. These forms should not be used when validating in secure applications.unparam: true
if warnings should be silenced for unused parameters.
Coding Conventions¶
Coding conventions include generic patterns that ensure the written code is consistently formatted.
Using two-space indentation¶
Tabs and 2-space indentation are being used equally. Since a lot of errors on JSLint often result from mixed use of space and tab, using 2 spaces throughout prevents these errors up front.
Good Example
function outer(a, b) {
var c = 1,
d = 2,
inner;
if (a > b) {
inner = function () {
return {
"r": c - d
};
};
} else {
inner = function () {
return {
"r": c + d
};
};
}
return inner;
}
Bad Example
function outer(a, b) {
var c = 1,
d = 2,
inner;
if (a > b) {
inner = function () {
return {
r: c - d
}}}};
Using shorthand for conditional statements¶
An alternative for using braces is the shorthand notation for conditional statements. When using multiple conditions, the conditional statement can be split on multiple lines.
Good Example
// single line
var results = test === true ? alert(1) : alert(2);
// multiple lines
var results = (test === true && number === undefined ?
alert(1) : alert(2));
var results = (test === true ?
alert(1) : number === undefined ?
alert(2) : alert(3));
Bad Example
// multiple conditions
var results = (test === true && number === undefined) ?
alert(1) :
alert(2);
Opening Brace Location¶
Always put the opening brace on the same line as the previous statement.
Bad Example
function func()
{
return
{
"name": "Batman"
};
}
Good Example
function func() {
return {
"name": "Batman"
};
}
Closing Brace Location¶
The closing brace should be on the same indent level as the original function call.
Bad Example
function func() {
return {
"name": "Batman"
};
}
Good Example
function func() {
return {
"name": "Batman"
};
}
Variable Declaration Location¶
Every variables should be declared at the top of its function.
Bad Example
function first() {
var a = 1, b = 2,
c, d;
// ...
}
function second() {
var a = 1, b = 2, c;
var d;
// ...
}
Good Example
function third() {
var a = 1, b = 2, c, d;
// ...
}
function fourth() {
var a = 1,
b = 2,
c,
d;
// ...
}
Function Declaration Location¶
Non anonymous functions should be declared before use and before every statements.
Bad Example
if (...) {
return {
"namedFunction": function namedFunction() { ... }
};
}
// or
if (...) {
function namedFunction() { ... }
return {
"namedFunction": namedFunction
};
}
Good Example
function namedFunction() { ... }
if (...) {
return {
"namedFunction": namedFunction
};
}
Anonymous Function Location¶
Execept if you want to keep your function without name, anonymous functions must not be declared in the same place as the variables.
Bad Example
function first() {
var a = 1, b = 2, func = function () {
return a;
};
// ...
}
Good Example
function second() {
var a = 1, b = 2;
function func() {
return a;
};
// ...
}
You can assign a variable to an anonymous function inside non-loop statements.
Bad Example
function third() {
var a = 1, b = 2, func;
for (...) {
b.forEach(function () { ... });
}
// ...
}
Good Example
function fourth() {
var a = 1, b = 2, func;
if (...) {
func = function () { ... };
} else {
func = function () { ... };
}
// ...
}
function fifth() {
var a = [], b = [];
function func() { ... }
for (...) {
b.forEach(func);
}
// ...
}
Naming Conventions¶
Naming conventions include generic patterns for setting names and identifiers throughout a script.
Constructors¶
Constructor functions (called with the new
statement) should always start with a capital letter:
// bad example
var test = new application();
// good example
var test = new Application();
Methods/Functions¶
A method/function should always start with a small letter.
// bad example
function MyFunction() {...}
// good example
function myFunction() {...}
TitleCase, camelCase¶
Follow the camel case convention, typing the words in lower-case, only capitalizing the first letter in each word.
// Good example constructor = TitleCase
var test = new PrototypeApplication();
// Bad example constructor
var test = new PROTOTYPEAPPLICATION();
// Good example functions/methods = camelCase
myFunction();
calculateArea();
// Bad example functions/methods
MyFunction();
CalculateArea();
Variables¶
Variable names with multiple words should always use an underscore between them.
// bad example
var deliveryNote = 1;
// good example
var delivery_note = 1;
Confusing variable names should end with the variable type.
// implicit type
var my_callback = doSomething();
var Person = require("./person");
// confusing names + var type
var do_something_function = doSomething.bind(context);
var value_list = getObjectOrArray();
// value_list can be an object which can be cast into an array
To use camelCase, when sometimes it is not possible to declare a function directly, the function variable name should match some pattern which shows that it is a function.
// good example
var doSomethingFunction = function () { ... };
// or
var tool = {"doSomething": function () { ... }};
// bad example
var doSomething = function () { ... };
Element Classes and IDs¶
JavaScript can access elements by their ID attribute and class names. When assigning IDs and class names with multiple words, these should also be separated by an underscore (same as variables).
Example
// bad example
test.setAttribute("id", "uniqueIdentifier");
// good example
test.setAttribute("id", "unique_identifier");
Discuss - checked with jQuery UI/jQuery Mobile, they don’t use written name conventions, only
- events names should fit their purpose (pageChange for changing a page)
- element classes use “-” like in ui-shadow
- “ui” should not be used by third party developers
- variables and events use lower camel-case like pageChange and activePage
Underscore Private Methods¶
Private methods should use a leading underscore to separate them from public methods (although this does not technically make a method private).
Good Example
var person = {
"getName": function () {
return this._getFirst() + " " + this._getLast();
},
"_getFirst": function () {
// ...
},
"_getLast": function () {
// ...
}
};
Bad Example
var person = {
"getName": function () {
return this.getFirst() + " " + this.getLast();
},
// private function
"getFirst": function () {
// ...
}
};
No Abbreviations¶
Abbreviations should not be used to avoid confusion.
Good Example
// delivery note
var delivery_note = 1;
Bad Example
// delivery note
var del_note = 1;
No Plurals¶
Plurals should not be used as variable names.
// good example
var delivery_note_list = ["one", "two"];
// bad example
var delivery_notes = ["one", "two"];
Use Comments¶
Comments should be used within reason but include enough information so that a reader can get a first grasp of what a part of code is supposed to do.
Good Example
var person = {
// returns full name string
"getName": function () {
return this._getFirst() + " " + this._getLast();
}
};
Bad Example
var person = {
"getName": function () {
return this._getFirst() + " " + this._getLast();
}
};
Documentation¶
You can use YUIDoc and its custom comment tags together with Node.js to generate the documentation from the script file itself. Comments should look like this:
Good Example
/**
* Reverse a string
*
* @param {String} input_string String to reverse
* @return {String} The reversed string
*/
function reverse(input_string) {
// ...
return output_string;
};
Bad Example
function reverse(input_string) {
// ...
return output_string;
};
Additional Readings¶
Resources, additional reading materials and links:
- JavaScript Patterns, main resource used.
- JSLint, code quality tool.
- JSLint Error Explanations, a useful reference.
- YUIDoc, generate documentation from code.