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Javascript/Coding Style: Difference between revisions

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{{draft}}
The Moodle JavaScript coding style.
{{Work in progress}}


This article is a proposal for a more codified coding style for JavaScript and will replace any existing coding styles after appropriate discussion, and consensus has been reached.
{{Deprecated|version=3.10}}


The Moodle JavaScript coding style
== Overview ==
 
This document outlines the exceptions to the general Moodle [[Coding_style|Coding style]] which apply to JavaScript.


== Overview ==
Unless otherwise specified, developers should follow the general coding style for advice on coding style.


Although most of the Moodle standard [[Coding_style|Coding style]] applies to JavaScript,
Note that some part of this document, specifically those making a reference to [[YUI|Yui]] modules, can be considered outdated. Since Moodle 2.9 all javascript is being moved to [[Javascript Modules|Javascript modules]] and, more recently, since Moodle 3.10, to ES6 Javascript modules (see MDLSITE-6130).
there are a number of exceptions. These are largely as a result of common
practice within the JavaScript community, or for increased clarity given different
terminology when dealing with frontend code (e.g. use of cartesian positions).


=== Scope ===
Please note that all new Javascript should be written using best practice ES-style javascript which is transpiled into ES2015 for backwards compatability. The use of jQuery and YUI in new code is not allowed, except where interfacing with legacy code which requires those objects and/or those features. Many parts of this document are specific to YUI and, where possible best practice modern JS should be written instead.


This document describes style guidelines for developers work on or with JavaScript code in Moodle.
This documented will be replaced in the future with an updated guide.


=== Goals ===
=== Goals ===
Line 34: Line 32:
=== Variable and function naming ===
=== Variable and function naming ===


Contrary to the standard Moodle coding style, we prefer to use camelCase
Contrary to the standard Moodle coding style, camelCase should be used to name variables and functions in JavaScript.
for JavaScript.
 
The justification for this is that:
* it is a commonly accepted practice within the wider JavaScript community;
* it is easier to read when dealing with variables describing cartesian points which are more common in JavaScript than PHP (e.g. currenty versus currentY); and
* it is the style used by the upstream YUI library for all variable names, and all CSS settings (keeping consistency within the codebase).


==== Correct ====
==== Correct ====
<code javascript>
<syntaxhighlight lang="javascript">
var currentY,
var currentY,
     courseCategory,
     courseCategory,
Line 60: Line 52:
     // Do stuff here.
     // Do stuff here.
};
};
</code>
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====
<code javascript>
<syntaxhighlight lang="javascript">
var current_y,
var current_y,
     currenty,
     currenty,
Line 95: Line 87:
}
}


</code>
</syntaxhighlight>


=== Class naming ===
=== Class naming ===


In line with common JavaScript practices, names of new classes being
Classes should be named using CamelCase starting with an uppercase
defined should be written using CamelCase starting with an uppercase
letter.
letter.


Line 107: Line 98:


==== Correct ====
==== Correct ====
<code javascript>
<syntaxhighlight lang="javascript">
// The instantiator:
// The instantiator:
function Pantry() {
function Pantry() {
Line 122: Line 113:


var myPantryShelf = new PantryShelf();
var myPantryShelf = new PantryShelf();
</code>
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====
<code javascript>
<syntaxhighlight lang="javascript">
// There is no distinction here between a normal function, and one used to
// There is no distinction here between a normal function, and one used to
// create a new object:
// create a new object:
Line 143: Line 134:
}
}
var myPantryShelf = new pantry_shelf();
var myPantryShelf = new pantry_shelf();
</code>
</syntaxhighlight>


=== Constants ===
=== Constants ===


We make use of a few constants in YUI modules, but all variables intended to be constants should use the same naming style of ALL UPPERCASE.
Variables intended to be constants should use the same naming style of ALL UPPERCASE.


We make frequent use of two constants in particular and recommend that they be used where appropriate in your code also:
Constants are recommended for use in the following scenarios:
* CSS: This is an object containing any CSS classes you may wish to use with Nodes; and
* CSS: An object containing any CSS classes you may wish to use with Nodes; and
* SELECTORS: This is an object containing query selectors for selecting Nodes.
* SELECTORS: An object containing query selectors for selecting Nodes.


Generally, the keys under this object should also be capitalised.
Generally, the keys under this object should also be capitalised.


==== Example ====
==== Example ====
<code javascript>
<syntaxhighlight lang="javascript">
var CSS = {
var CSS = {
         MYCLASS: 'myclass',
         MYCLASS: 'myclass',
Line 171: Line 162:
         .removeClass(CSS.MYCLASS);
         .removeClass(CSS.MYCLASS);
}
}
</code>
</syntaxhighlight>


== Variables ==
== Variables ==


As usual with JavaScript, all variables must:
All variables must:
* be declared before they are used and using the <tt>var</tt> keyword;
* be declared before they are used and using the <tt>var</tt> keyword;
* be declared once, and only once, for the scope in which they are used;
* be declared once, and only once, for the scope in which they are used;
Line 181: Line 172:
* use sensible naming, following the naming convention.
* use sensible naming, following the naming convention.


=== Number of variable declarations ===
=== Method call wrapping ===
 
The number of variable declarations should be kept to a sensible minimum,
though this interpretation is left largely up to the individual developer.
 
Where a set of variables is separated by block of code, it is reasonable to
logically separate the sets of variables around the code block.
 
==== Correct ====
 
<code javascript>
var oneVariable,
    twoVariable,
    threeVariable,
    more;
 
// Some other code goes here which breaks up the logical flow
oneVariable = Y.Node.Create('<div></div>');
twoVariable = Y.one('#someid');
 
// Some more variable definitions relating to the next set of code goes
// here.
var anotherVariable,
    andAnother;
</code>
 
==== Incorrect ====
 
<code javascript>
// Only one variable definition should be used:
var oneVariable;
var twoVariable;
var threeVariable;
var more;
</code>
 
=== Initial values ===
 
Variable definitions describing variables which have initial content
should:
* fit on a single line in the case of simple types; and
* be spread across multiple lines for complex types (including '''all''' objects and arrays, no matter how simple) - this includes those which are empty.
 
The reason for this is primarily to help preserve revision and change
history for different lines of code in a sensible and meaningful fashion.
 
==== Correct ====
 
<code javascript>
var someBoolean = true,
    someNumber = 10,
    anObjectWithoutContent = {
    },
    anObjectWithContent = {
        exampleContent: 'value'
    };
</code>
==== Incorrect ====
 
<code javascript>
var anObjectWithContent = {},
    anObjectWithContent = {exampleContent: 'value')};
</code>
 
 
=== Object keys ===
 
When working with objects, object property names should not be quoted unless they are a reserved word.
 
==== Correct ====
 
<code javascript>
var anObjectWithContent = {
    exampleContent: 'value',
    "this": 'that'
};
</code>
==== Incorrect ====
 
<code javascript>
var anObjectWithContent = {
    "exampleContent": 'value'
};
 
</code>
 
== Functions ==
 
Function definitions must meet the following rules:
* they must be appropriately named according to the [[#Naming_conventions|naming schemes]];
* they must be documented using standard YUIDoc tags;
* they must not declare variables which they do not intend to use within the function body;
** unless the argument is before an actively used argument in the function declaration;
* variables defined in the function declaration must not be re-declared within the function;
* they should have an appropriated return value:
** where appropriate they should return the current object and declare themselves chainable.
 
=== Return values ===
 
Where it makes sense, and no return value is already required, functions should return the current object and describe themselves as chainable in the YUIDoc docblock.
 
When setting the @chainable tag, it is not necessary to set a @return value too.
 
This enables developers to call a number of functions in a sensible, chained fashion. For example, the YUI Node module defines many chainable functions so it is possible to call a number of functions on a single instance:
 
<code javascript>
Y.Node.create('<div />')
    .addClass('someClass')
    .setAttribute('foo', 'bar')
    .setData('baz, 'qux');
</code>
 
=== Examples ===
==== Correct ====
<code javascript>
/**
* Do something with the two values provided and return something else.
*
* @method doSomething
* @param optionOne {Number} The first parameter
* @param unusedMiddleArgument {Number} This parameter is not used in this
* function definition, but perhaps it's provided by the event calling it
* so we have to have it and cannot remove it.
* @param optionTwo {Number} The second parameter
* @return Number The result of a really complex mathematical equation
* which you couldn't possible do your own
*/
var doSomething = function(optionOne, unusedMiddleArgument, optionTwo) {
    return optionOne + optionTwo;
};
</code>
 
==== Incorrect ====
<code javascript>
// Undocumented function:
var doSomething = function(anArgument, anotherArgument, aThirdArgument) {
    // The second argument is re-declared:
    var anotherArgument = anArgument + 1;
 
    // The third argument is never used:
    return anArgument + anotherArgument;
};
</code>
 
== Line length ==
 
We use the same [https://docs.moodle.org/dev/Coding_style#Maximum_Line_Length | maximum line guidelines as our php standards].
=== Line wrapping ===
 
==== Statement wrapping ====
 
When wrapping a long line as part of a statement, indent the following line
by 8 spaces rather than 4. For example:
 
<code javascript>
if (someObject.hasClass(CSS.CLASSTOTEST) &&
        someOtherObject.hasClass(CSS.CLASSTOTEST)) {
    return true;
}
</code>
 
==== Method call wrapping ====


When wrapping a long line which consists of a chained series of functions,
When wrapping a long line which consists of a chained series of functions,
Line 348: Line 178:
a new line.
a new line.


The line should be indented by 4 spaces.
The line should be indented by spaces.


The start of each line should contain the concatanation character, and the
The start of each line should contain the concatanation character, and the
final line should contain a trailing semicolon.
final line should contain a trailing semicolon.


===== Correct =====
==== Correct ====
<code javascript>
<syntaxhighlight lang="javascript">
var childNode = Y.Node.create('<div />')
var childNode = Y.Node.create('<div />')
    .addClass(CSS.SOMECLASS)
        .addClass(CSS.SOMECLASS)
    .setAttribute('someAttribute', 'someValue')
        .setAttribute('someAttribute', 'someValue')
    .appendTo(parentNode);
        .appendTo(parentNode);
</code>
</syntaxhighlight>


===== Incorrect =====
==== Incorrect ====
<code javascript>
<syntaxhighlight lang="javascript">
// All on one line:
// All on one line:
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS).setAttribute('someAttribute', 'someValue').appendTo(parentNode);
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS).setAttribute('someAttribute', 'someValue').appendTo(parentNode);
Line 368: Line 198:
// A mix of separation and line concatanation:
// A mix of separation and line concatanation:
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS)
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS)
    .setAttribute('someAttribute', 'someValue').appendTo(parentNode);
        .setAttribute('someAttribute', 'someValue').appendTo(parentNode);


// The concatanation character is at the end of the line:
// The concatanation character is at the end of the line:
var childNode = Y.Node.create('<div />').
var childNode = Y.Node.create('<div />').
    addClass(CSS.SOMECLASS).
        addClass(CSS.SOMECLASS).
    setAttribute('someAttribute', 'someValue').
        setAttribute('someAttribute', 'someValue').
    appendTo(parentNode);
        appendTo(parentNode);
</code>
</syntaxhighlight>


== Whitespace ==
== Whitespace ==
Line 400: Line 230:


==== Correct ====
==== Correct ====
<code javascript>
<syntaxhighlight lang="javascript">
// Valid binary operators:
// Valid binary operators:
var a = 1,
var a = 1,
Line 428: Line 258:
}
}


</code>
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====
<code javascript>
<syntaxhighlight lang="javascript">
var a=1,
var a=1,
     b= (a&&1),
     b= (a&&1),
Line 446: Line 276:
for ( index = 0;index<a; index ++ ) {
for ( index = 0;index<a; index ++ ) {
}
}
</code>
</syntaxhighlight>


=== Assignation ===
=== Assignment ===
In the case of object property assignation, there should be a space after
In the case of object property assignment, there should be a space after
the colon, but not before.
the colon, but not before.


==== Correct ====
==== Correct ====
<code javascript>
<syntaxhighlight lang="javascript">
var anObject = {
var anObject = {
    someKey: 'someValue',
        someKey: 'someValue',
    anotherKey: Y.one(SELECTORS.FOO)
        anotherKey: Y.one(SELECTORS.FOO)
};
    };
</code>
</syntaxhighlight>
 


==== Incorrect ====
==== Incorrect ====
<code javascript>
<syntaxhighlight lang="javascript">
var anObject = {
var anObject = {
    // Incorrect because a space is present both before and after the assignation character:
        // Incorrect because a space is present both before and after the assignation character:
    someKey : 'someValue',
        someKey : 'someValue',


    // Incorrect because there is no whitespace either side of the assignation character:
        // Incorrect because there is no whitespace either side of the assignation character:
    anotherKey:Y.one(SELECTORS.FOO)
        anotherKey:Y.one(SELECTORS.FOO)
};
    };
</code>
</syntaxhighlight>


== Documentation and comments ==
== Documentation and comments ==


We are attempting to document all YUI modules that we write and as such largely follow the upstream YUI guidance which is available at http://yui.github.io/yuidoc/syntax/index.html.
Modules should be documented using valid [https://jsdoc.app/ JSDoc].
 
Documentation can be generated locally by running <nowiki>grunt jsdoc</nowiki>. This will generate documentation in the <nowiki>/jsdoc</nowiki> directory at the root of your Moodle site where it can be accessed in your browser by viewing http://[wwwroot]/jsdoc.


=== General notes ===
=== General notes ===
Line 488: Line 319:
* use the correct JavaScript types where relevant (note, Int is not a valid type in JavaScript);
* use the correct JavaScript types where relevant (note, Int is not a valid type in JavaScript);
* use all appropriate tags;
* use all appropriate tags;
* produce valid documentation using the YUIDoc toolset;
* produce valid documentation using the jsdoc toolset;
* have a linebreak between the description and the list of tags.
* have a linebreak between the description and the list of tags.


Note:
A few notes:
* Docblocks should not make use of the <nowiki>@package [name]</nowiki> tag as this is not valid in JSDoc.
* JS only has <nowiki>number</nowiki>, and does not have separate types for <nowiki>int</nowiki> and <nowiki>float</nowiki>.
 
==== Correct ====


YUIDoc will only generate documentation for docblocks starting with /**.
<syntaxhighlight lang="javascript">
/**
* All ES and AMD modules should have a module directive with the AMD module name.
*
* For a module located at question/type/example/amd/src/dothethings.js the module name will be as follows.
*
* @module qtype_example/dothethings
*/


YUIDoc will try to generate documentation for *all* docblocks starting /**.
/**
* If the module defines a class, it should be documented with the class tag.
*
* If the class is the default export for the module, then the name specified should match the module name.
*
* @class qtype_example/dothethings
*/
export default class {
  // ...
}


==== Correct ====
/**
* If the module defines multiple classes then they should be namespaced under the module name.
*
* @class qtype_example/dothethings/MyClass
*/
export const MyClass = class {
  // ...
}


<code javascript>
/**
/**
  * This is an example docblock comment. It describes a function called
  * This is an example docblock comment. It describes a function called
Line 506: Line 363:
  * It adds a number of jars of marmite to the cupboard.
  * It adds a number of jars of marmite to the cupboard.
  *
  *
  * @method addMarmite
  * @method
  * @param {Number} [jarCount=1] The number of jars of marmite to add to the
  * @param {Number} [jarCount=1] The number of jars of marmite to add to the
  * cupboard. This parameter is optional and defaults to 1.
  * cupboard. This parameter is optional and defaults to 1.
* @chainable
  */
  */
</code>
export const addMarmite = (jarCount = 1) => {
  // ...
};
 
/**
* This docblock describes the property weight, in grams.
*
* @property {number} weight
*/
export const weight = 400;
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====


<code javascript>
<syntaxhighlight lang="javascript">
/*
/*
  * This is an invalid comment block. It wouldn't be picked up by yuidoc as
  * This block would not be picked up by jsdoc as it only has a single * in the opening line.
* the comment style is incorrect.
  *
  *
  * @method foo
  * @method foo
  */
  */


// This is also an invalid comment block and wouldn't be picked up by
// This comment would not be picked up by jsdoc either.
// YUIDoc.
</syntaxhighlight>
 
 
=== General comments ===
 
All shorter comments, for example those explaining the subsequent few lines of code should use the // style of comments.
 
==== Correct ====
 
<syntaxhighlight lang="javascript">
// This is a valid set of comments for one line.
 
// And this is a valid longer comment to describe the subsequent few lines
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.
</syntaxhighlight>
 
==== Incorrect ====
 
<syntaxhighlight lang="javascript">
/* This is an invalid comment style for short comments. */
 
//This is also an invalid style as there is no leading whitespace after the
//comment indicator.


/**
/**
* Although this style would be picked up by YUIDoc, it is hard to read.
* This is an invalid multi-line comment. Multi-line comments should not
*
* use the docblock style comments unless they are a valid and fully
* @method foo
* formatted docblock.
*/
*/
 
/*
* This is an also invalid multi-line comment. Although it is not a full
* docblock style, it does not start with the // style of comment
* indicator.
*/
</syntaxhighlight>
 
== Documentation and comments ==
 
Modules should be documented using valid [https://jsdoc.app/ JSDoc].
 
Documentation can be generated locally by running <nowiki>grunt jsdoc</nowiki>. This will generate documentation in the <nowiki>/jsdoc</nowiki> directory at the root of your Moodle site where it can be accessed in your browser by viewing http://[wwwroot]/jsdoc.
 
=== General notes ===
 
* Unless otherwise specified, comments should conform to the general style guidelines;
* all comments must start with leading whitespace before the first word on each line; and
* all indentation must be in addition to any existing leading whitespace on the line.
 
=== Official documentation ===
 
All JavaScript documentation must:
* use the correct docblock format;
* use the correct JavaScript types where relevant (note, Int is not a valid type in JavaScript);
* use all appropriate tags;
* produce valid documentation using the YUIDoc toolset;
* have a linebreak between the description and the list of tags.
 
A few notes:
* Docblocks should not make use of the <nowiki>@package [name]</nowiki> tag as this is not valid in JSDoc.
* JS only has <nowiki>number</nowiki>, and does not have separate types for <nowiki>int</nowiki> and <nowiki>float</nowiki>.
 
==== Correct ====
 
<syntaxhighlight lang="javascript">
/**
* All ES and AMD modules should have a module directive with the AMD module name.
*
* For a module located at question/type/example/amd/src/dothethings.js the module name will be as follows.
*
* @module qtype_example/dothethings
*/
 
/**
* If the module defines a class, it should be documented with the class tag.
*
* If the class is the default export for the module, then the name specified should match the module name.
*
* @class qtype_example/dothethings
*/
export default class {
  // ...
}
 
/**
* If the module defines multiple classes then they should be namespaced under the module name.
*
* @class qtype_example/dothethings/MyClass
*/
export const MyClass = class {
  // ...
}


/**
/**
  *Although this style would be picked up by YUIDoc, it is also hard to read.
  * This is an example docblock comment. It describes a function called
* marmite.
*
* It adds a number of jars of marmite to the cupboard.
  *
  *
  *@method foo
  * @method
* @param {Number} [jarCount=1] The number of jars of marmite to add to the
* cupboard. This parameter is optional and defaults to 1.
  */
  */
export const addMarmite = (jarCount = 1) => {
  // ...
};


/**
/**
  * This docblock is mostly valid but does not include a linebreak between
  * This docblock describes the property weight, in grams.
  * the description, and the tags.
*
* @property {number} weight
*/
export const weight = 400;
</syntaxhighlight>
 
==== Incorrect ====
 
<syntaxhighlight lang="javascript">
/*
* This block would not be picked up by jsdoc as it only has a single * in the opening line.
  *
  * @method foo
  * @method foo
  */
  */
</code>
 
// This comment would not be picked up by jsdoc either.
</syntaxhighlight>




Line 554: Line 526:
==== Correct ====
==== Correct ====


<code javascript>
<syntaxhighlight lang="javascript">
// This is a valid set of comments for one line.
// This is a valid set of comments for one line.


Line 560: Line 532:
// in as much detail as required. It can consist of multiple sentences, as
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.
// long as each new line starts with the correct comment style.
</code>
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====


<code javascript>
<syntaxhighlight lang="javascript">
/* This is an invalid comment style for short comments. */
/* This is an invalid comment style for short comments. */


Line 581: Line 553:
  * indicator.
  * indicator.
  */
  */
</code>
</syntaxhighlight>


== Syntax ==
=== General comments ===


=== Comparison operators ===
All shorter comments, for example those explaining the subsequent few lines of code should use the // style of comments.


Strict comparisons should always be used.
Comments not intended for official documentation must *not* use the Docblock style of commenting as YUIDoc will attempt to include the comment in official documentation.


==== Correct ====
==== Correct ====
<code javascript>
var a = 1,
    b = "1";
if (a === b) {
}


var c = (a === b);
<syntaxhighlight lang="javascript">
</code>
// This is a valid set of comments for one line.
 
// And this is a valid longer comment to describe the subsequent few lines
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.
</syntaxhighlight>
 
==== Incorrect ====
 
<syntaxhighlight lang="javascript">
/* This is an invalid comment style for short comments. */
 
//This is also an invalid style as there is no leading whitespace after the
//comment indicator.
 
/**
* This is an invalid multi-line comment. Multi-line comments should not
* use the docblock style comments unless they are a valid and fully
* formatted docblock.
*/
 
/*
* This is an also invalid multi-line comment. Although it is not a full
* docblock style, it does not start with the // style of comment
* indicator.
*/
</syntaxhighlight>
 
== Promises ==
Promises are used extensively in modern Moodle Javascript APIs to handle asynchronous situations. It is unfortunately common to misunderstand how they operate and introduce bugs which only expose themselves in asynchronous edge cases (see article [https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html we have a problem with promises]). To encourage promise usage to be more understandable, consistent and avoid edge case bugs, we have adopted best practices suggested by Nolan Lawson and verified by [https://github.com/xjamundx/eslint-plugin-promise eslint-plugin-promise].
 
=== Always return or throw  ===
 
When writing promises they must:
* return another promise, or
* return a synchronous value (or undefined), or
* throw a synchronous error
 
==== Correct ====
<syntaxhighlight lang="javascript">
str.get_strings(stringRequests).then(function(title) {
    return templates.renderPix(image, 'core', title);
}).then(function(pixhtml) {
    $('#selector').html(pixhtml);
    makeUIVisible();
    return;
});
</syntaxhighlight>
 
==== Incorrect ====
<syntaxhighlight lang="javascript">
str.get_strings(stringRequests).then(function(title) {
    templates.renderPix(image, 'core', title).function(pixhtml) {
        $('#selector').html(pixhtml);
    });
}).then(function() {
    // Wrong because renderPix() has not guaranted to be resolved here.
    makeUIVisible();
});
</syntaxhighlight>
 
=== Chain rather than nest ===
 
Promises as are a construct which help prevent the <i>pyramid of doom</i> and regain linear control flow and error handling. They should not be nested.
 
* Promises should be chained rather than nested.
* $.when() should be used to deal with the result of multiple promises
 
==== Correct ====
<syntaxhighlight lang="javascript">
// Single promise chain:
str.get_string('title')
.then(function(title) {
    return templates.renderPix(image, 'core', title);
}).then(function(pixhtml) {
    actionitem.find('.icon').replaceWith(pixhtml);
    return;
});
 
// Need the result of two promises at once:
$.when(Str.get_string('competencypicker', 'tool_lp'), renderPromise())
.then(function(title, html) {
    self._popup = new Dialogue(title, html);
});
</syntaxhighlight>


==== Incorrect ====
==== Incorrect ====


<code javascript>
<syntaxhighlight lang="javascript">
var a = 1,
str.get_string('title')
     b = "1";
.then(function(title) {
if (a == b) {
    return templates.renderPix(image, 'core', title).then(function(pixhtml) {
        actionitem.find('.icon').replaceWith(pixhtml);
        return;
    });
});
 
return renderPromise().then(function (html) {
    return Str.get_string('competencypicker', 'tool_lp').then(function(title) {
        self._popup = new Dialogue(title, html);
     });
});
</syntaxhighlight>
 
=== Avoid mixing callbacks and promises ===
Avoid mixing callbacks and promises. Design code to embrace promise-y patterns for asynchronous code to make maximum use of promises.
 
==== Correct ====
<syntaxhighlight lang="javascript">
function doWork(input) {
    return renderPromise();
}
}


var c = (a == b);
doWork(input).then(function () {
</code>
  $('#selector').html(html);
  return;
}).catch(Notification.exception);
</syntaxhighlight>
 
==== Incorrect ====
<syntaxhighlight lang="javascript">
function doWork(input, successHandler, errorHandler) {
    renderPromise().then(function (html) {
        successHandler(html)
    }).catch(errorHandler);
}


== To be documented ==
doWork(input, function () {
  $('#selector').html(html);
}, function (error){
  Notification.exception(error);
});
</syntaxhighlight>


== See Also ==
== See Also ==


* [[Coding_style|Coding Style]]
* [[CSS_Coding_Style|CSS Coding Style]]
* [[Coding]]
* [[Coding]]
* [[Coding_style|Coding style]]
 
[[Category:Coding guidelines]]
[[Category:Javascript]]

Latest revision as of 04:17, 4 October 2021

The Moodle JavaScript coding style.

This feature has been marked as deprecated since Moodle 3.10


Overview

This document outlines the exceptions to the general Moodle Coding style which apply to JavaScript.

Unless otherwise specified, developers should follow the general coding style for advice on coding style.

Note that some part of this document, specifically those making a reference to Yui modules, can be considered outdated. Since Moodle 2.9 all javascript is being moved to Javascript modules and, more recently, since Moodle 3.10, to ES6 Javascript modules (see MDLSITE-6130).

Please note that all new Javascript should be written using best practice ES-style javascript which is transpiled into ES2015 for backwards compatability. The use of jQuery and YUI in new code is not allowed, except where interfacing with legacy code which requires those objects and/or those features. Many parts of this document are specific to YUI and, where possible best practice modern JS should be written instead.

This documented will be replaced in the future with an updated guide.

Goals

Consistent coding style is important in any development project, and particularly when many developers are involved. A standard style helps to ensure that the code is easier to read and understand, which helps overall quality.

Abstract goals we strive for:

  • simplicity
  • readability
  • tool friendliness

Naming conventions

Variable and function naming

Contrary to the standard Moodle coding style, camelCase should be used to name variables and functions in JavaScript.

Correct

var currentY,
    courseCategory,
    lastValue,
    lastBackgroundColor;

function doSomething() {
    // Do stuff here.
}

function doSomethingElse() {
    // Do stuff here.
}

var someFunction = function() {
    // Do stuff here.
};

Incorrect

var current_y,
    currenty,
    course_category,
    coursecategory,
    last_value,
    lastvalue,
    last_background_color,
    lastbackgroundcolor;

function dosomething() {
}

function do_something_else() {
}

var somefunction = function() {
};

var some_other_function = function() {
};

var somevalue = null;

if (someTest) {
    somevalue = function() {
        return (something && complicated || somethingelse);
    };
} else {
    somevalue = 'basicvalue';
}

Class naming

Classes should be named using CamelCase starting with an uppercase letter.

This helps to clearly separate variables, and standard functions from those used to create a new instance.

Correct

// The instantiator:
function Pantry() {
    // Setup code goes here.
}

// Making use of it:
var myPantry = new Pantry();


// And another:
function PantryShelf() {
}

var myPantryShelf = new PantryShelf();

Incorrect

// There is no distinction here between a normal function, and one used to
// create a new object:
function pantry() {
}

// This results in an unclear object creation:
var myPantry = new pantry();

// This one is also incorrect, despite using camelCase:
function pantryShelf() {
}
var myPantryShelf = new pantryShelf();

// This one is also incorrect:
function pantry_shelf() {
}
var myPantryShelf = new pantry_shelf();

Constants

Variables intended to be constants should use the same naming style of ALL UPPERCASE.

Constants are recommended for use in the following scenarios:

  • CSS: An object containing any CSS classes you may wish to use with Nodes; and
  • SELECTORS: An object containing query selectors for selecting Nodes.

Generally, the keys under this object should also be capitalised.

Example

var CSS = {
        MYCLASS: 'myclass',
        YOURCLASS: 'yourclass'
    },
    SELECTORS = {
        MYNODES: 'div.example .myclass',
        YOURNODES: 'div.example .yourclass'
    };

function anExampleFunction() {
    theNode = Y.one(SELECTORS.MYNODES)
        .addClass(CSS.YOURCLASS)
        .removeClass(CSS.MYCLASS);
}

Variables

All variables must:

  • be declared before they are used and using the var keyword;
  • be declared once, and only once, for the scope in which they are used;
  • only be declared if they are to be used; and
  • use sensible naming, following the naming convention.

Method call wrapping

When wrapping a long line which consists of a chained series of functions, break the line at the end of each function, and continue the next chain on a new line.

The line should be indented by spaces.

The start of each line should contain the concatanation character, and the final line should contain a trailing semicolon.

Correct

var childNode = Y.Node.create('<div />')
        .addClass(CSS.SOMECLASS)
        .setAttribute('someAttribute', 'someValue')
        .appendTo(parentNode);

Incorrect

// All on one line:
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS).setAttribute('someAttribute', 'someValue').appendTo(parentNode);

// A mix of separation and line concatanation:
var childNode = Y.Node.create('<div />').addClass(CSS.SOMECLASS)
        .setAttribute('someAttribute', 'someValue').appendTo(parentNode);

// The concatanation character is at the end of the line:
var childNode = Y.Node.create('<div />').
        addClass(CSS.SOMECLASS).
        setAttribute('someAttribute', 'someValue').
        appendTo(parentNode);

Whitespace

Operators

There should be a space at either side of all binary operators to help improve legibility of code. This includes:

  • =
  • &&
  • ||
  • ===
  • +
  • -
  • /
  • *

There should be no space around unary operators. This includes:

  • !
  • ++
  • --

There should be no space around the function operator (.)

Correct

// Valid binary operators:
var a = 1,
    b = (a && 1),
    c = (b || 1),
    d = (b === c),
    e = Y.Node.create('<div>Some Content</div>');

// No space around the . operator when it's not a continuation:
e.someFunctionCall();

// Whitespace is allowed for a function operator when it is a continuation starting on a new line:
e.someFunction()
    .someOtherFunction()
    .someFinalFunction();

// Unary operators should not be separated by whitespace:
a = a++;
b = b--;
c = (!e.someResult());

// An example bringing most of these together:
var index,
    loopTest = 0;
for (index = 0; (!loopTest <= (a / b * (c + d - e.getValue()))); index++) {
    loopTest = index * 12;
}

Incorrect

var a=1,
    b= (a&&1),
    c =(b||1),
    d = (b===c);

a = a ++;
b =b++;
c= c++;
d = d++ ;

var e = Y . Node . create('<div>Some content</div>');

for ( index = 0;index<a; index ++ ) {
}

Assignment

In the case of object property assignment, there should be a space after the colon, but not before.

Correct

var anObject = {
        someKey: 'someValue',
        anotherKey: Y.one(SELECTORS.FOO)
    };

Incorrect

var anObject = {
        // Incorrect because a space is present both before and after the assignation character:
        someKey : 'someValue',

        // Incorrect because there is no whitespace either side of the assignation character:
        anotherKey:Y.one(SELECTORS.FOO)
    };

Documentation and comments

Modules should be documented using valid JSDoc.

Documentation can be generated locally by running grunt jsdoc. This will generate documentation in the /jsdoc directory at the root of your Moodle site where it can be accessed in your browser by viewing http://[wwwroot]/jsdoc.

General notes

  • Unless otherwise specified, comments should conform to the general style guidelines;
  • all comments must start with leading whitespace before the first word on each line; and
  • all indentation must be in addition to any existing leading whitespace on the line.

Official documentation

All JavaScript documentation must:

  • use the correct docblock format;
  • use the correct JavaScript types where relevant (note, Int is not a valid type in JavaScript);
  • use all appropriate tags;
  • produce valid documentation using the jsdoc toolset;
  • have a linebreak between the description and the list of tags.

A few notes:

  • Docblocks should not make use of the @package [name] tag as this is not valid in JSDoc.
  • JS only has number, and does not have separate types for int and float.

Correct

/**
 * All ES and AMD modules should have a module directive with the AMD module name.
 *
 * For a module located at question/type/example/amd/src/dothethings.js the module name will be as follows.
 *
 * @module qtype_example/dothethings
 */

/**
 * If the module defines a class, it should be documented with the class tag.
 *
 * If the class is the default export for the module, then the name specified should match the module name.
 *
 * @class qtype_example/dothethings
 */
export default class {
  // ...
}

/**
 * If the module defines multiple classes then they should be namespaced under the module name.
 *
 * @class qtype_example/dothethings/MyClass
 */
export const MyClass = class {
  // ...
}

/**
 * This is an example docblock comment. It describes a function called
 * marmite.
 *
 * It adds a number of jars of marmite to the cupboard.
 *
 * @method
 * @param {Number} [jarCount=1] The number of jars of marmite to add to the
 * cupboard. This parameter is optional and defaults to 1.
 */
export const addMarmite = (jarCount = 1) => {
  // ...
};

/**
 * This docblock describes the property weight, in grams.
 *
 * @property {number} weight
 */
export const weight = 400;

Incorrect

/*
 * This block would not be picked up by jsdoc as it only has a single * in the opening line.
 *
 * @method foo
 */

// This comment would not be picked up by jsdoc either.


General comments

All shorter comments, for example those explaining the subsequent few lines of code should use the // style of comments.

Correct

// This is a valid set of comments for one line.

// And this is a valid longer comment to describe the subsequent few lines
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.

Incorrect

/* This is an invalid comment style for short comments. */

//This is also an invalid style as there is no leading whitespace after the
//comment indicator.

/**
 * This is an invalid multi-line comment. Multi-line comments should not
 * use the docblock style comments unless they are a valid and fully
 * formatted docblock.
 */

/*
 * This is an also invalid multi-line comment. Although it is not a full
 * docblock style, it does not start with the // style of comment
 * indicator.
 */

Documentation and comments

Modules should be documented using valid JSDoc.

Documentation can be generated locally by running grunt jsdoc. This will generate documentation in the /jsdoc directory at the root of your Moodle site where it can be accessed in your browser by viewing http://[wwwroot]/jsdoc.

General notes

  • Unless otherwise specified, comments should conform to the general style guidelines;
  • all comments must start with leading whitespace before the first word on each line; and
  • all indentation must be in addition to any existing leading whitespace on the line.

Official documentation

All JavaScript documentation must:

  • use the correct docblock format;
  • use the correct JavaScript types where relevant (note, Int is not a valid type in JavaScript);
  • use all appropriate tags;
  • produce valid documentation using the YUIDoc toolset;
  • have a linebreak between the description and the list of tags.

A few notes:

  • Docblocks should not make use of the @package [name] tag as this is not valid in JSDoc.
  • JS only has number, and does not have separate types for int and float.

Correct

/**
 * All ES and AMD modules should have a module directive with the AMD module name.
 *
 * For a module located at question/type/example/amd/src/dothethings.js the module name will be as follows.
 *
 * @module qtype_example/dothethings
 */

/**
 * If the module defines a class, it should be documented with the class tag.
 *
 * If the class is the default export for the module, then the name specified should match the module name.
 *
 * @class qtype_example/dothethings
 */
export default class {
  // ...
}

/**
 * If the module defines multiple classes then they should be namespaced under the module name.
 *
 * @class qtype_example/dothethings/MyClass
 */
export const MyClass = class {
  // ...
}

/**
 * This is an example docblock comment. It describes a function called
 * marmite.
 *
 * It adds a number of jars of marmite to the cupboard.
 *
 * @method
 * @param {Number} [jarCount=1] The number of jars of marmite to add to the
 * cupboard. This parameter is optional and defaults to 1.
 */
export const addMarmite = (jarCount = 1) => {
  // ...
};

/**
 * This docblock describes the property weight, in grams.
 *
 * @property {number} weight
 */
export const weight = 400;

Incorrect

/*
 * This block would not be picked up by jsdoc as it only has a single * in the opening line.
 *
 * @method foo
 */

// This comment would not be picked up by jsdoc either.


General comments

All shorter comments, for example those explaining the subsequent few lines of code should use the // style of comments.

Comments not intended for official documentation must *not* use the Docblock style of commenting as YUIDoc will attempt to include the comment in official documentation.

Correct

// This is a valid set of comments for one line.

// And this is a valid longer comment to describe the subsequent few lines
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.

Incorrect

/* This is an invalid comment style for short comments. */

//This is also an invalid style as there is no leading whitespace after the
//comment indicator.

/**
 * This is an invalid multi-line comment. Multi-line comments should not
 * use the docblock style comments unless they are a valid and fully
 * formatted docblock.
 */

/*
 * This is an also invalid multi-line comment. Although it is not a full
 * docblock style, it does not start with the // style of comment
 * indicator.
 */

General comments

All shorter comments, for example those explaining the subsequent few lines of code should use the // style of comments.

Comments not intended for official documentation must *not* use the Docblock style of commenting as YUIDoc will attempt to include the comment in official documentation.

Correct

// This is a valid set of comments for one line.

// And this is a valid longer comment to describe the subsequent few lines
// in as much detail as required. It can consist of multiple sentences, as
// long as each new line starts with the correct comment style.

Incorrect

/* This is an invalid comment style for short comments. */

//This is also an invalid style as there is no leading whitespace after the
//comment indicator.

/**
 * This is an invalid multi-line comment. Multi-line comments should not
 * use the docblock style comments unless they are a valid and fully
 * formatted docblock.
 */

/*
 * This is an also invalid multi-line comment. Although it is not a full
 * docblock style, it does not start with the // style of comment
 * indicator.
 */

Promises

Promises are used extensively in modern Moodle Javascript APIs to handle asynchronous situations. It is unfortunately common to misunderstand how they operate and introduce bugs which only expose themselves in asynchronous edge cases (see article we have a problem with promises). To encourage promise usage to be more understandable, consistent and avoid edge case bugs, we have adopted best practices suggested by Nolan Lawson and verified by eslint-plugin-promise.

Always return or throw

When writing promises they must:

  • return another promise, or
  • return a synchronous value (or undefined), or
  • throw a synchronous error

Correct

str.get_strings(stringRequests).then(function(title) {
    return templates.renderPix(image, 'core', title);
}).then(function(pixhtml) {
    $('#selector').html(pixhtml);
    makeUIVisible();
    return;
});

Incorrect

str.get_strings(stringRequests).then(function(title) {
    templates.renderPix(image, 'core', title).function(pixhtml) {
        $('#selector').html(pixhtml);
    });
}).then(function() {
    // Wrong because renderPix() has not guaranted to be resolved here.
    makeUIVisible();
});

Chain rather than nest

Promises as are a construct which help prevent the pyramid of doom and regain linear control flow and error handling. They should not be nested.

  • Promises should be chained rather than nested.
  • $.when() should be used to deal with the result of multiple promises

Correct

// Single promise chain:
str.get_string('title')
.then(function(title) {
    return templates.renderPix(image, 'core', title);
}).then(function(pixhtml) {
    actionitem.find('.icon').replaceWith(pixhtml);
    return;
});

// Need the result of two promises at once:
$.when(Str.get_string('competencypicker', 'tool_lp'), renderPromise())
.then(function(title, html) {
    self._popup = new Dialogue(title, html);
});

Incorrect

str.get_string('title')
.then(function(title) {
    return templates.renderPix(image, 'core', title).then(function(pixhtml) {
        actionitem.find('.icon').replaceWith(pixhtml);
        return;
    });
});

return renderPromise().then(function (html) {
    return Str.get_string('competencypicker', 'tool_lp').then(function(title) {
        self._popup = new Dialogue(title, html);
    });
});

Avoid mixing callbacks and promises

Avoid mixing callbacks and promises. Design code to embrace promise-y patterns for asynchronous code to make maximum use of promises.

Correct

function doWork(input) {
     return renderPromise();
}

doWork(input).then(function () {
   $('#selector').html(html);
   return;
}).catch(Notification.exception);

Incorrect

function doWork(input, successHandler, errorHandler) {
     renderPromise().then(function (html) {
         successHandler(html)
    }).catch(errorHandler);
}

doWork(input, function () {
   $('#selector').html(html);
}, function (error){
  Notification.exception(error);
});

See Also