This is a set of coding conventions and rules for use in JavaScript programming. It is inspired by the Sun document Code Conventions for the Java Programming Language. It is heavily modified of course because JavaScript is not Java.
The long-term value of software to an organization is in direct proportion to the quality of the codebase. Over its lifetime, a program will be handled by many pairs of hands and eyes. If a program is able to clearly communicate its structure and characteristics, it is less likely that it will break when modified in the never-too-distant future.
Code conventions can help in reducing the brittleness of programs.
All of our JavaScript code is sent directly to the public. It should always be of publication quality.
Neatness counts.
JavaScript programs should be stored in and delivered as .js
files.
JavaScript code should not be embedded in HTML files unless the code is specific to a single session. Code in HTML adds significantly to pageweight with no opportunity for mitigation by caching and compression.
<script src=filename.js>
tags should be placed as late in the body as possible. This reduces
the effects of delays imposed by script loading on other page
components. There is no need to use the language
or type attributes. It is the server,
not the script tag, that determines the MIME type.
The unit of indentation is four spaces. Use of tabs should be avoided because (as of this writing in the 21st Century) there still is not a standard for the placement of tabstops. The use of spaces can produce a larger filesize, but the size is not significant over local networks, and the difference is eliminated by minification.
Avoid lines longer than 80 characters. When a statement will not fit on a single line, it may be necessary to break it. Place the break after an operator, ideally after a comma. A break after an operator decreases the likelihood that a copy-paste error will be masked by semicolon insertion. The next line should be indented 8 spaces.
Be generous with comments. It is useful to leave information that will be read at a later time by people (possibly yourself) who will need to understand what you have done. The comments should be well-written and clear, just like the code they are annotating. An occasional nugget of humor might be appreciated. Frustrations and resentments will not.
It is important that comments be kept up-to-date. Erroneous comments can make programs even harder to read and understand.
Make comments meaningful. Focus on what is not immediately visible. Don't waste the reader's time with stuff like
i = 0; // Set i to zero.
Generally use line comments. Save block comments for formal documentation.
All variables should be declared before used. JavaScript does not require this, but doing so makes the program easier to read and makes it easier to detect undeclared variables that may become implied globals. Implied global variables should never be used. Use of global variables should be minimized.
The var statement should be the first
statement in the function body.
It is preferred that each variable be given its own line and comment. They should be listed in alphabetical order if possible.
var currentEntry, // currently selected table entry
level, // indentation level
size; // size of tableJavaScript does not have block scope, so defining variables in blocks can confuse programmers who are experienced with other C family languages. Define all variables at the top of the function.
All functions should be declared before they are used. Inner
functions should follow the var
statement. This helps make it clear what variables are included in
its scope.
There should be no space between the name of a function and the (
(left parenthesis) of
its parameter list. There should be one space between the )
(right parenthesis) and
the { (left
curly brace) that begins the statement body. The body itself
is indented four spaces. The } (right
curly brace) is aligned with the line containing the beginning
of the declaration of the function.
function outer(c, d) {
var e = c * d;
function inner(a, b) {
return (e * a) + b;
}
return inner(0, 1);
}This convention works well with JavaScript because in JavaScript, functions and object literals can be placed anywhere that an expression is allowed. It provides the best readability with inline functions and complex structures.
function getElementsByClassName(className) {
var results = [];
walkTheDOM(document.body, function (node) {
var array, // array of class names
ncn = node.className; // the node's classname
// If the node has a class name, then split it into a list of simple names.
// If any of them match the requested name, then append the node to the list of results.
if (ncn && ncn.split(' ').indexOf(className) >= 0) {
results.push(node);
}
});
return results;
}
If a function literal is anonymous, there should be one space between
the word function and the (
(left parenthesis). If
the space is omited, then it can appear that the function's name is
function, which is an incorrect reading.
div.onclick = function (e) {
return false;
};
that = {
method: function () {
return this.datum;
},
datum: 0
};Use of global functions should be minimized.
When a function is to be invoked immediately, the entire invocation expression should be wrapped in parens so that it is clear that the value being produced is the result of the function and not the function itself.
var collection = (function () {
var keys = [], values = [];
return {
get: function (key) {
var at = keys.indexOf(key);
if (at >= 0) {
return values[at];
}
},
set: function (key, value) {
var at = keys.indexOf(key);
if (at < 0) {
at = keys.length;
}
keys[at] = key;
values[at] = value;
},
remove: function (key) {
var at = keys.indexOf(key);
if (at >= 0) {
keys.splice(at, 1);
values.splice(at, 1);
}
}
};
}());Names should be formed from the 26 upper and lower case letters (A
.. Z, a ..
z), the 10 digits (0
.. 9), and _
(underbar). Avoid use
of international characters because they may not read well or be
understood everywhere. Do not use $
(dollar sign) or \
(backslash) in names.
Do not use _ (underbar)
as the first or last character of a name. It is sometimes intended to
indicate privacy, but it does not actually provide privacy. If
privacy is important, use the forms that provide private
members. Avoid conventions that demonstrate a lack of competence.
Most variables and functions should start with a lower case letter.
Constructor functions that must be used with the new
prefix should start with a capital letter. JavaScript issues
neither a compile-time warning nor a run-time warning if a required
new is omitted. Bad things can happen if
new is not used, so the capitalization
convention is the only defense we have.
Global variables should be in all caps. (JavaScript does not have macros or constants, so there isn't much point in using all caps to signify features that JavaScript doesn't have.)
Each line should contain at most one statement. Put a ;
(semicolon) at the end
of every simple statement. Note that an assignment statement that is
assigning a function literal or object literal is still an assignment
statement and must end with a semicolon.
JavaScript allows any expression to be used as a statement. This can mask some errors, particularly in the presence of semicolon insertion. The only expressions that should be used as statements are assignments and invocations.
Compound statements are statements that contain lists of
statements enclosed in { } (curly
braces).
The enclosed statements should be indented four more spaces.
The {
(left curly brace)
should be at the end of the line that begins the compound statement.
The }
(right curly brace)
should begin a line and be indented to align with the beginning of
the line containing the matching {
(left curly brace).
Braces should be used around all statements, even single
statements, when they are part of a control structure, such as an if
or for statement. This makes it easier
to add statements without accidentally introducing bugs.
Statement labels are optional. Only these statements should be
labeled: while, do,
for, switch.
return
Statement
A return statement with a value
should not use ( ) (parentheses)
around the value. The return value expression must start on the same
line as the return keyword in order to
avoid semicolon insertion.
if
Statement
The if class of statements should
have the following form:
if
(condition)
{statements
}
if
()
{statements
}
else {
}
if
()
{statements
}
else if ()
{statements
}
else {
}
for
Statement
A for class of statements should have
the following form:
for
(initialization; condition;
update)
{statements
}
for
( in object)
{filter
if ()
{statements
}
}
The first form should be used with arrays and with loops of a predeterminable number of iterations.
The second form should be used with objects. Be aware that members
that are added to the prototype of the object will be
included in the enumeration. It is wise to program defensively by
using the hasOwnProperty method to
distinguish the true members of the object:
for
(variable in object)
{object
if
(.hasOwnProperty(variable))
{statements
}
}
while
Statement
A while statement should have the
following form:
while
(condition)
{statements
}
do
Statement
A do statement should have the
following form:
do
{statements
}
while ();
Unlike the other compound statements, the do
statement always ends with a ;
(semicolon).
switch
Statement
A switch statement should have the
following form:
switch
(expression) {expression
case
:statements
default:
}
Each case is aligned with the switch.
This avoids over-indentation. A case
label is not a statement, and should not be indented like one.
Each group of statements (except the default)
should end with break, return,
or throw. Do not fall through.
try
Statement
The try class of statements should
have the following form:
try
{
statements
}
catch (variable)
{
statements
}
try {
statements
}
catch (variable)
{
statements
}
finally {
statements
}
continue
Statement
Avoid use of the continue statement.
It tends to obscure the control flow of the function.
with
Statement
The with statement should
not be used.
Blank lines improve readability by setting off sections of code that are logically related.
Blank spaces should be used in the following circumstances:
A keyword followed by ( (left
parenthesis) should be separated by a space.
while (true) {
A blank space should not be used
between a function value and its (
(left parenthesis).
This helps to distinguish between keywords and function invocations.
All binary operators except .
(period) and (
(left parenthesis) and
[ (left
bracket) should be separated from their operands by a space.
No space should separate a unary
operator and its operand except when the operator is a word such as
typeof.
Each ; (semicolon)
in the control part of a for statement
should be followed with a space.
Whitespace should follow every , (comma).
{} and []
Use {} instead of new
Object(). Use [] instead of new
Array().
Use arrays when the member names would be sequential integers. Use objects when the member names are arbitrary strings or names.
, (comma)
Operator
Avoid the use of the comma operator. (This does not apply to the
comma separator, which is used in object literals, array literals,
var statements, and parameter lists.)
In JavaScript blocks do not have scope. Only functions have scope. Do not use blocks except as required by the compound statements.
Avoid doing assignments in the condition part of if
and while statements.
Is
if (a = b) {a correct statement? Or was
if (a == b) {intended? Avoid constructs that cannot easily be determined to be correct.
=== and !==
Operators.
Use the === and !==
operators. The == and !=
operators do type coercion and should not be used.
Be careful to not follow a + with +
or ++. This pattern can be confusing.
Insert parens between them to make your intention clear.
total = subtotal + +myInput.value;
is better written as
total = subtotal + (+myInput.value);
so that the + + is not misread as ++.
eval is Evil
The eval function is the most misused
feature of JavaScript. Avoid it.
eval has aliases. Do not use the
Function constructor. Do not pass
strings to setTimeout or setInterval.