Code Conventions

Programming language style guides are important for the long-term maintainability of software. This guide is based on the Code Conventions for the Java Programming Language and Douglas Crockford's Code Conventions for the JavaScript Programming Language. Modifications have been made due to my personal experience and preferences.

File Format

Each file has this same basic format:

/**
 * @fileoverview Description of the file
 * @author Your Name
 * @copyright 2015 Your Name. All rights reserved.
 * See LICENSE file in root directory for full license.
 */

"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------

// require() statements

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

// private methods/data

//------------------------------------------------------------------------------
// Public Interface
//------------------------------------------------------------------------------

// exported objects/methods
module.exports = {

};

The @author field gives you credit for having created the file. The @copyright field indicates that you are the copyright holder for the file.

Your submission may touch other parts of the ESLint code that you did not write. In that case, you are welcome to add a copyright notice to the top of the file if you have done any amount of significant work on the file (we leave it up to you to decide what "significant" means - if you aren't sure, just ask). You should never change the @author field, but you can add another @copyright field on top of the existing ones, such as:

/**
 * @fileoverview Description of the file
 * @author Author's Name
 * @copyright 2015 Your Name. All rights reserved.
 * @copyright 2014 Author's Name. All rights reserved.
 * See LICENSE file in root directory for full license.
 */

Indentation

Each indentation level is made up of four spaces. Do not use tabs.

// Good
if (true) {
    doSomething();
}

Primitive Literals

Strings should always use double quotes (never single quotes) and should always appear on a single line. Never use a slash to create a new line in a string.

// Good
var name = "Nicholas";

// Bad: Single quotes
var name = 'Nicholas';

// Bad: Wrapping to second line
var longString = "Here's the story, of a man \
named Brady.";

Numbers should be written as decimal integers, e-notation integers, hexadecimal integers or floating-point decimals with at least one digit before and one digit after the decimal point. Never use octal literals.

// Good
var count = 10;

// Good
var price = 10.0;
var price = 10.00;

// Good
var num = 0xA2;

// Good
var num = 1e23;

// Bad: Hanging decimal point
var price = 10.;

// Bad: Leading decimal point
var price = .1;

// Bad: Octal (base 8) is deprecated
var num = 010;

The special value null should be used only in the following situations:

  1. To initialize a variable that may later be assign an object value.
  2. To compare against an initialized variable that may or may not have an object value.
  3. To pass into a function where an object is expected.
  4. To return from a function where an object is expected.

Examples:

// Good
var person = null;

// Good
function getPerson() {
    if (condition) {
        return new Person("Nicholas");
    } else {
        return null;
    }
}

// Good
var person = getPerson();
if (person !== null){
    doSomething();
}

// Bad: Testing against uninitialized variable
var person;
if (person != null){
    doSomething();
}

// Bad: Testing to see if an argument was passed
function doSomething(arg1, arg2, arg3, arg4){
    if (arg4 != null){
        doSomethingElse();
    }
}

Never use the special value undefined. To see if a variable has been defined, use the typeof operator:

// Good
if (typeof variable == "undefined") {
    // do something
}

// Bad: Using undefined literal
if (variable == undefined) {
    // do something
}

Operator Spacing

Operators with two operands must be preceded and followed by a single space to make the expression clear. Operators include assignments and logical operators.

// Good
var found = (values[i] === item);

// Good
if (found && (count > 10)) {
    doSomething();
}

// Good
for (i = 0; i < count; i++) {
    process(i);
}

// Bad: Missing spaces
var found = (values[i]===item);

// Bad: Missing spaces
if (found&&(count>10)) {
    doSomething();
}

// Bad: Missing spaces
for (i=0; i<count; i++) {
    process(i);
}

Parentheses Spacing

When parentheses are used, there should be no white space immediately after the opening paren or immediately before the closing paren.

// Good
var found = (values[i] === item);

// Good
if (found && (count > 10)) {
    doSomething();
}

// Good
for (i = 0; i < count; i++) {
    process(i);
}

// Bad: Extra space after opening paren
var found = ( values[i] === item);

// Bad: Extra space before closing paren
if (found && (count > 10) ) {
    doSomething();
}

// Bad: Extra space around argument
for (i = 0; i < count; i++) {
    process( i );
}

Object Literals

Object literals should have the following format:

Examples:

// Good
var object = {

    key1: value1,
    key2: value2,

    func: function() {
        // do something
    },

    key3: value3
};

// Bad: Improper indentation
var object = {
                key1: value1,
                key2: value2
            };

// Bad: Missing blank lines around function
var object = {

    key1: value1,
    key2: value2,
    func: function() {
        // do something
    },
    key3: value3
};

When an object literal is passed to a function, the opening brace should be on the same line as if the value is a variable. All other formatting rules from above still apply.

// Good
doSomething({
    key1: value1,
    key2: value2
});

// Bad: All on one line
doSomething({ key1: value1, key2: value2 });

Comments

Make frequent use of comments to aid others in understanding your code. Use comments when:

Single-Line Comments

Single-line comments should be used to documentation one line of code or a group of related lines of code. A single-line comment may be used in three ways:

  1. On a separate line, describing the code beneath it.
  2. At the end of a line, describing the code before it.
  3. On multiple lines, to comment out sections of code.

When on a separate line, a single-line comment should be at the same indentation level as the code it describes and be preceded by a single line. Never use multiple single-line comments on consecutive lines, use a multi-line comment instead.

// Good
if (condition){

    // if you made it here, then all security checks passed
    allowed();
}

// Bad: No empty line preceding comment
if (condition){
    // if you made it here, then all security checks passed
    allowed();
}

// Bad: Wrong indentation
if (condition){

// if you made it here, then all security checks passed
    allowed();
}

// Bad: This should be a multi-line comment
// This next piece of code is quite difficult, so let me explain.
// What you want to do is determine if the condition is true
// and only then allow the user in. The condition is calculated
// from several different functions and may change during the
// lifetime of the session.
if (condition){
    // if you made it here, then all security checks passed
    allowed();
}

For single-line comments at the end of a line, ensure there is at least one indentation level between the end of the code and the beginning of the comment:

// Good
var result = something + somethingElse;    // somethingElse will never be null

// Bad: Not enough space between code and comment
var result = something + somethingElse;// somethingElse will never be null

The only acceptable time to have multiple single-line comments on successive lines is to comment out large sections of code. Multi-line comments should not be used for this purpose.

// Good
// if (condition){
//    doSomething();
//    thenDoSomethingElse();
// }

Multi-Line Comments

Multi-line comments should be used to document code that requires more explanation. Each multi-line comment should have at least three lines:

  1. The first line contains only the /* comment opening. No further text is allowed on this line.
  2. The next line(s) have a * aligned with the * in the first line. Text is allowed on these lines.
  3. The last line has the */ comment opening aligned with the preceding lines. No other text is allowed on this line.

The first line of multi-comments should be indented to the same level as the code it describes. Each subsequent line should have the same indentation plus one space (for proper alignment of the * characters). Each multi-line comment should be preceded by one empty line.

// Good
if (condition){

    /*
     * if you made it here,
     * then all security checks passed
     */
    allowed();
}

// Bad: No empty line preceding comment
if (condition){
    /*
     * if you made it here,
     * then all security checks passed
     */
    allowed();
}

// Bad: Missing a space after asterisk
if (condition){

    /*
     *if you made it here,
     *then all security checks passed
     */
    allowed();
}

// Bad: Wrong indentation
if (condition){

/*
 * if you made it here,
 * then all security checks passed
 */
    allowed();
}

// Bad: Don't use multi-line comments for trailing comments
var result = something + somethingElse;    /*somethingElse will never be null*/

Comment Annotations

Comments may be used to annotate pieces of code with additional information. These annotations take the form of a single word followed by a colon. The acceptable annotations are:

These annotations may be used with either single-line or multi-line comments and should follow the same formatting rules as the general comment type. Examples:

// Good
// TODO: I'd like to find a way to make this faster
doSomething();

// Good
/*
 * HACK: Have to do this for IE. I plan on revisiting in
 * the future when I have more time. This probably should
 * get replaced before v1.2.
 */
if (document.all) {
    doSomething();
}

// Good
// REVIEW: Is there a better way to do this?
if (document.all) {
    doSomething();
}

// Bad: Annotation spacing is incorrect
// TODO : I'd like to find a way to make this faster
doSomething();

// Bad: Comment should be at the same indentation as code
    // REVIEW: Is there a better way to do this?
if (document.all) {
    doSomething();
}

Variable Declarations

All variables should be declared before they are used. Variable declarations should take place at the beginning of a function using a single var statement with one variable per line. All lines after the first should be indented one level so the variable names line up. Variables should be initialized when declared if applicable and the equals operator should be at a consistent indentation level. Initialized variables should come first followed by uninitialized variables.

// Good
var count   = 10,
    name    = "Nicholas",
    found   = false,
    empty;

// Bad: Improper initialization alignment
var count = 10,
    name = "Nicholas",
    found= false,
    empty;

// Bad: Incorrect indentation
var count   = 10,
name    = "Nicholas",
found   = false,
empty;

// Bad: Multiple declarations on one line
var count   = 10, name = "Nicholas",
    found   = false, empty;

// Bad: Uninitialized variables first
var empty,
    count   = 10,
    name    = "Nicholas",
    found   = false;

// Bad: Multiple var statements
var count   = 10,
    name    = "Nicholas";

var found   = false,
    empty;

Always declare variables. Implied globals should not be used.

Function Declarations

Functions should be declared before they are used. When a function is not a method (not attached to an object) it should be defined using function declaration format (not function expression format nor using the Function constructor). There should be no space between the function name and the opening parentheses. There should be one space between the closing parentheses and the right brace. The right brace should be on the same line as the function keyword. There should be no space after the opening parentheses or before the closing parentheses. Named arguments should have a space after the comma but not before it. The function body should be indented one level.

// Good
function doSomething(arg1, arg2) {
    return arg1 + arg2;
}

// Bad: Improper spacing of first line
function doSomething (arg1, arg2){
    return arg1 + arg2;
}

// Bad: Function expression
var doSomething = function(arg1, arg2) {
    return arg1 + arg2;
};

// Bad: Left brace on wrong line
function doSomething(arg1, arg2)
{
    return arg1 + arg2;
}

// Bad: Using Function constructor
var doSomething = new Function("arg1", "arg2", "return arg1 + arg2");

Functions declared inside of other functions should be declared immediately after the var statement.

// Good
function outer() {

    var count   = 10,
        name    = "Nicholas",
        found   = false,
        empty;

    function inner() {
        // code
    }

    // code that uses inner()
}

// Bad: Inner function declared before variables
function outer() {

    function inner() {
        // code
    }

    var count   = 10,
        name    = "Nicholas",
        found   = false,
        empty;

    // code that uses inner()
}

Anonymous functions may be used for assignment of object methods or as arguments to other functions. There should be no space between the function keyword and the opening parentheses.

// Good
object.method = function() {
    // code
};

// Bad: Incorrect spacing
object.method = function () {
    // code
};

Immediately-invoked functions should surround the entire function call with parentheses.

// Good
var value = (function() {

    // function body

    return {
        message: "Hi"
    }
}());

// Bad: No parentheses around function call
var value = function() {

    // function body

    return {
        message: "Hi"
    }
}();

// Bad: Improper parentheses placement
var value = (function() {

    // function body

    return {
        message: "Hi"
    }
})();

Naming

Care should be taken to name variables and functions properly. Names should be limited to alphanumeric characters and, in some cases, the underscore character. Do not use the dollar sign ($) or back slash (\) characters in any names.

Variable names should be formatted in camel case with the first letter being lowercase and the first letter of each subsequent word being uppercase. The first word of a variable name should be a noun (not a verb) to avoid confusion with functions. Do not use underscore for variable names.

// Good
var accountNumber = "8401-1";

// Bad: Begins with uppercase letter
var AccountNumber = "8401-1";

// Bad: Begins with verb
var getAccountNumber = "8401-1";

// Bad: Uses underscore
var account_number = "8401-1";

Function names should also be formatted using camel case. The first word of a function name should be a verb (not a noun) to avoid confusion with variables. Do not use underscore for function names.

// Good
function doSomething() {
    // code
}

// Bad: Begins with uppercase letter
function DoSomething() {
    // code
}

// Bad: Begins with noun
function car() {
    // code
}

// Bad: Uses underscores
function do_something() {
    // code
}

Constructor functions, those functions used with the new operator to create new objects, should be formatted in camel case but must begin with an uppercase letter. Constructor function names should begin with a non-verb because new is the action of creating an object instance.

// Good
function MyObject() {
    // code
}

// Bad: Begins with lowercase letter
function myObject() {
    // code
}

// Bad: Uses underscores
function My_Object() {
    // code
}

// Bad: Begins with verb
function getMyObject() {
    // code
}

Variables that act as constants (values that won't be changed) should be formatted using all uppercase letters with words separated by a single underscore.

// Good
var TOTAL_COUNT = 10;

// Bad: Camel case
var totalCount = 10;

// Bad: Mixed case
var total_COUNT = 10;

Object properties follow the same naming conventions as variables. Object methods follow the same naming conventions as functions. If a property or method is meant to be private, then it should be prefixed with an underscore character.

// Good
var object = {
    _count: 10,

    _getCount: function () {
        return this._count;
    }
};

Strict Mode

Strict mode should be used only inside of functions and never globally.

// Bad: Global strict mode
"use strict";

function doSomething() {
    // code
}

// Good
function doSomething() {
    "use strict";

    // code
}

If you want strict mode to apply to multiple functions without needing to write "use strict" multiple times, use immediate function invocation:

// Good
(function() {
    "use strict";

    function doSomething() {
        // code
    }

    function doSomethingElse() {
        // code
    }

}());

Assignments

When assigning a value to a variable, use parentheses around a right-side expression that contains a comparison.

// Good
var flag = (i < count);

// Bad: Missing parentheses
var flag = i < count;

Equality Operators

Use === and !== instead of == and !=. This avoids type coercion errors.

// Good
var same = (a === b);

// Bad: Using ==
var same = (a == b);

Ternary Operator

The ternary operator should be used only for assigning values conditionally and never as a shortcut for an if statement.

// Good
var value = condition ? value1 : value2;

// Bad: no assignment, should be an if statement
condition ? doSomething() : doSomethingElse();

Statements

Simple Statements

Each line should contain at most one statement. All simple statements should end with a semicolon (;).

// Good
count++;
a = b;

// Bad: Multiple statements on one line
count++; a = b;

return Statement

A return statement with a value should not use parentheses unless they make the return value more obvious in some way. Example:

return;

return collection.size();

return (size > 0 ? size : defaultSize);

Compound Statements

Compound statements are lists of statements enclosed inside of braces.

if Statement

The if class of statements should have the following form:

if (condition) {
    statements
}

if (condition) {
    statements
} else {
    statements
}

if (condition) {
    statements
} else if (condition) {
    statements
} else {
    statements
}

It is never permissible to omit the braces in any part of an if statement.

// Good
if (condition) {
    doSomething();
}

// Bad: Improper spacing
if(condition){
    doSomething();
}

// Bad: Missing braces
if (condition)
    doSomething();

// Bad: All on one line
if (condition) { doSomething(); }

// Bad: All on one line without braces
if (condition) doSomething();

for Statement

The for class of statements should have the following form:

for (initialization; condition; update) {
    statements
}

for (variable in object) {
    statements
}

Variables should not be declared in the initialization section of a for statement.

// Good
var i,
    len;

for (i=0, len=10; i < len; i++) {
    // code
}

// Bad: Variables declared during initialization
for (var i=0, len=10; i < len; i++) {
    // code
}

// Bad: Variables declared during initialization
for (var prop in object) {
    // code
}

When using a for-in statement, double-check whether or not you need to use hasOwnProperty() to filter out object members.

while Statement

The while class of statements should have the following form:

while (condition) {
    statements
}

do Statement

The do class of statements should have the following form:

do {
    statements
} while (condition);

Note the use of a semicolon as the final part of this statement. There should be a space before and after the while keyword.

switch Statement

The switch class of statements should have the following form:

switch (expression) {
    case expression:
        statements

    default:
        statements
}

Each case is indented one level under the switch. Each case after the first, including default, should be preceded by a single empty line.

Each group of statements (except the default) should end with break, return, throw, or a comment indicating fall through.

// Good
switch (value) {
    case 1:
        /* falls through */

    case 2:
        doSomething();
        break;

    case 3:
        return true;

    default:
        throw new Error("This shouldn't happen.);
}

If a switch doesn't have a default case, then it should be indicated with a comment.

// Good
switch (value) {
    case 1:
        /*falls through*/

    case 2:
        doSomething();
        break;

    case 3:
        return true;

    // no default
}

try Statement

The try class of statements should have the following form:

try {
    statements
} catch (variable) {
    statements
}

try {
    statements
} catch (variable) {
    statements
} finally {
    statements
}

White Space

Blank lines improve readability by setting off sections of code that are logically related.

Two blank lines should always be used in the following circumstances:

One blank line should always be used in the following circumstances:

Blank spaces should be used in the following circumstances:

Things to Avoid