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JavaScript standards

These standards are written for module components, but parts can apply equally to non-component use cases.

Browser support

Product developers are encouraged to include Origami JavaScript using a ‘cuts the mustard’ test to allow module developers to assume a minimum standard of support for JavaScript. Minimum browser support standards for JavaScript are yet to be agreed, and are currently the subject of issue 88.

Encapsulation rules

  • Add no objects to the global scope, other than JSONp callback function names. Variables declared outside of any enclosing function are permitted, provided that the module requires a commonJS interface. If you don’t want to depend on CommonJS, wrap the module in an IIFE.
  • If the module does not require CommonJS it must include a Universal Module Definition that includes support for CommonJS.
  • Do not read or modify the DOM on parse
  • If it’s possible for the module to create DOM nodes, timers, or otherwise occupy more than a token amount of memory, it should export a destroy method that reverts the module to a pre-init state. This is not necessary for large modules like o-header where it doesn’t make sense for it to be reverted
  • Do not leave any non-garbage collectable traces after destroy is called
  • Do not modify the DOM outside of areas of owned DOM, except:
    • to add feature flag CSS classes to the documentElement; or
    • to add a new section of owned DOM to an element explicitly nominated by the host application (e.g. by the host application calling a method of the module’s API and passing an element to which the module is asked to append its DOM)
  • Do not require global variables to be defined prior to the script loading. If your module requires configuration, read the config from data attributes attached to parts of DOM that your module will own (see Data attributes for details)
  • Do not assume the existence of globals except those defined as part of ECMAScript 5 and features listed in the browserFeatures/required section of origami.json.

Scoping and binding this

The value of this should not be copied into non-semantic variables such as that, self or _this in order to embed a child function context. Instead, either use a semantic name, or bind the correct value of this. Some object methods accept the intended value of this as an argument, such as Array.prototype.filter, and this method should be considered most preferred:

arr.filter(function(item) {
}, this);

As an alternative, use bind:

  .then(function(response) {
    return response.json();

If you do copy a reference to this into a separate variable, make it semantic:

var post = this;


Modules must do as little as possible on parse, instead deferring start-up tasks to a publicly exported, static ‘init’ function that should be either invoked explicitly using the module’s API, or automatically by binding to the o.DOMContentLoaded or o.load events.

Where modules bind to the o.DOMContentLoaded or o.load events, their init method must be callable with no arguments, that is, they may accept arguments, but if they do, all such arguments must be optional (see issue 228).

Modules that expose an init method or an instance constructor which takes an argument identifying an area of owned DOM must allow all of the following types of references:

  • An HTMLElement object
  • A string containing a valid querySelector expression, e.g. “.main-content > [data-o-component~=’o-share’]”
  • Nothing (or any falsey value), which should be interpreted as document.body

Where this reference is passed to an init function, the module may create multiple instances and return them in an array. Where passed to a constructor, the module must only create one instance and return it.

Where the reference is to an element that is not itself owned DOM, the init function may traverse the subtree looking for elements that are.

Where JavaScript exists to enhance elements, and accompanying CSS depends on knowing whether the JavaScript intends to apply (or has applied) that enhancement, the JavaScript may add a data attribute of the form data-{modulename}-js with no value to the root element of the component when the JavaScript initialises. For example, o-tabs markup would not contain a o--if-js class, because the tabs content should remain visible even if the tabs JavaScript is not running on the page, but if the JavaScript does run, it could apply an data-o-tabs-js data attribute to allow the tabs CSS to hide all but the selected tab panel.

Error handling

Modules should use o-errors to report runtime JavaScript errors and exceptions, as well as log notices and other significant events, using the oErrors.log custom event.


If a module’s JavaScript requires configuration, the following methods of passing that configuration must be supported.

Data attributes on owned DOM

If a module acts to enhance markup, the module must be configurable using data- attributes on the HTML element that is the root element of the DOM owned by the module. Data attributes must be named data-{modulename}-{key}, e.g. data-o-share-id. The module may also create attributes of this form at runtime, provided that the element is already within owned DOM for that module.

Global declarative config block

Where it is possible for multiple instances of a module to exist on a page and for the same configuration to apply to all of them, or where a module has no markup (e.g. o-tracking or o-errors), the module must support declarative configuration via JSON data placed within a <script> block with a type='application/json' and a data attribute in the module’s namespace with the key ‘config’ and no value, ie. data-{modulename}-config. For example:

<script data-o-errors-config type='application/json'>
        "sentryEndpoint": "https://....",
        "application": {
            "version": "1.2.3",
            "name": "Foo Application"

Components must parse any such configuration using JSON.parse and only in response to an event (such as o.DOMContentLoaded) or function call. Components must not expect more than one global declarative config block in their namespace to be present on the page.

Any configuration option expecting a function must not be defined in a declarative config block and must be optional, providing default behaviour where the imperative configuration using init() is absent. If a configuration key is present in the declarative config block that expects a function, an Error should be thrown to warn the developer of the invalid configuration.

DOM Selectors

When using selector engines other than native querySelector, modules must not use selectors that are incompatible with querySelector. This allows for an easier future upgrade path to querySelector.

Modules may assume that any HTML markup that relates to their component follows the hierarchical structure specified in their module’s Mustache template. However, modules should not make assumptions about the order of HTML elements, and should, as far as possible, cope with the presence within the component of elements not specified in the template.

Modules must not throw an error if there are no instances of the module’s owned DOM in the page.

Communicating with host page code and other components

Modules may wish to communicate (or make communication possible) with other components of the same type, other components of different types, or non-component code in the host page. This should be accomplished with API methods (when invoking known dependencies) and DOM events (in all other cases).


Modules may expose an external API via module.exports. Modules must not rely on the existence of any APIs other than those that are explicitly required dependencies, expressed in the browserFeatures property of origami.json, or defined as part of the DOM level 2 specification.


Modules may emit events to allow loose coupling with other components and the host page. In doing so, the module must:

  • use only browser-native DOM events with bubbling enabled
  • specify createevent as a required browser feature in the browserFeatures section of origami.json
  • where the module wishes to attach custom data payloads to events, specify customevents as a required browser feature in addition to ‘createevent’, use the CustomEvent API, and pass an object in the details property.
  • trigger events only on elements within the component’s owned DOM, or otherwise only on the body element
  • namespace event names with the name of the module in camelCase, separated from the event name with a dot, e.g. oModuleName.eventName
  • name the event using the present tense, e.g. dialogClose, not dialogClosed, and using camel-case.

A valid example of a module emitting a DOM event is shown below:

this.dispatchEvent(new CustomEvent('oTestModule.oTestClick', {
  detail: {...},
  bubbles: true

Modules may bind to events emitted by themselves, other modules, the host page or the browser (except DOMContentLoaded and load). In doing so, the module:

  • must not stop the propagation chain except for events created by itself
  • should bind only to the BODY element and use event delegation to ensure that handlers do not need to be bound every time elements are created. If not bound to the body element, handlers must be bound to elements within the module’s owned DOM.

Modules should handle events during the bubbling phase, not the capturing phase (unless the event has no bubbling phase)

If a module wishes to bind to the DOMContentLoaded or load browser events, it must prefix the event name with o., and must expose the function that it binds to the event via its external API, eg:

document.addEventListener('o.DOMContentLoaded', init);
exports.init = init;

Foreign events

Modules may emit events defined by other modules, using the other module’s namespace, but must only do so if:

  • the foreign module is not a direct dependency; and
  • there are no callbacks in the event details payload; and
  • the foreign module has invited public use of the event in its documentation and has provided a comprehensive spec for the details payload

For the most part, use of this technique creates too much ‘magic’ behaviour that would not be expected by a product developer and should be avoided, but in some cases e.g. analytics, may be a reasonable compromise to enable loose coupling.

Use of the z-axis (o-layers)

A module e.g. o-overlays and o-hiearchical-nav, may need to display some or all of its owned DOM outside of the normal content flow so that it obscures content outside its owned DOM. The module must bind to and fire o-layers events on its closest parent with the class o-layers__context, or body if no such element exists. The module must use the custom events defined in o-layers to:

  • broadcast changes in its own state
  • listen for events fired in its o-layers__context by other modules that make use of the z-axis

Any module may use the o-layers__context class to define a new region of the DOM that can handle new layers independently of other regions of the DOM (e.g. two graphs handling their own tooltips independently, a date-picker appearing within a modal dialog).

Data storage

Modules that store data on the client via user-agent APIs must encapsulate all the logic required to get and set that data and must remain compatible with the format of data that they store, unless the major version number of the module changes. In that case the module must not invalidate any existing data, and should provide advice in docs on migrating user data from previous versions.


Modules should avoid containing functions with more than 3 arguments. Where more parameters are required, consider passing an object (and if so, consider using lo-dash’s defaults function).


Object properties must not be named after reserved words in the JavaScript language. (Learn more)

Object prototypes must not be overwritten. Instead, assign additonal properties to the prototype individually:

Jedi.prototype.fight = function fight() {

Overwriting the prototype wipes out the constructor property and makes inheritance difficult. An exception to this is when creating subclasses, in which case there is no alternative, but the constructor property should be reinstated (read more):

function Super(x) { ... }
Super.prototype.foo = ...

function Sub(x, y) {
    Sub.superclass.constructor.call(this, x);
Sub.superclass = Super.prototype;
Sub.prototype = Object.create(Sub.superclass);
Sub.prototype.constructor = Sub;


Modules must not animate elements using methods that do not utilise hardware acceleration if hardware accelerated alternatives are available. For example, repositioning an element repeatedly using its left or top CSS properties is not allowed. Instead, use CSS transitions and will-change. On user agents that do not support accelerated animation, animation should not be used.

Feature stability

JavaScript in modules may use ES6. Modules must not use JavaScript features that are not yet part of a finalised standard, or which are proprietary, even if polyfills for them are available.

Product developers are expected to transpile ES6 syntax to be ES5 compatible if required. Currently Origami build tools does this automatically in all cases using BabelJS, with the assumption that no browser supports the ES6 syntax natively.

Syntax convention rules

JavaScript must be linted with ESLint. If you wish to specify a particular ESLint configuration you may do so at the module level with a .eslintrc file, and at the file level with a /*eslint ... */ comment. If you specify neither of these, code must pass a ESLint check with the following settings:

Developers should stick to the above .eslintrc config, since this represents a common standard across FT teams, but are permitted to make changes if desired. In addition to the ESLint rules:


Single line comments should be placed on a newline above the subject of the comment. An empty line should be inserted before the comment.


JavaScript modules in Origami components may want to load additional files (fonts, JSON data, images etc) that are also part of the component’s file tree. To resolve these paths safely, JS modules wishing to load sub-resources from their own component must resolve the file path using the Origami assets module:

someiframe.src = require('o-assets').resolve('/img/logo.png', 'tracking');

Without any explicit configuration, o-assets will assume, as we do for sub-resources in Sass, that the modules are installed publicly at a URL path of /bower_components on the current host, and will form URLs on that basis. Product developers are advised to reconfigure o-assets to accommodate their own server-side URL routing architecture.

Where external resources are not within Origami modules, a protocol-relative URL must be used (see issue 173).

Inlining subresources

In some cases it may be desirable or necessary to include the content of a static asset in a JavaScript source bundle (typically to include templates). To do this, use the textrequireify loader for webpack, which provides a requireText method. The standard Origami build process includes this, so it is available through the build service (learn more).

You would write this in your JavaScript source:

var template = requireText('main.mustache');

And it would be converted to this by the build process:

var template = 'This is the content of main.mustache';

Hover events

Where JavaScript includes bindings to mouse... hover events (such as mouseover, mouseout, mouseenter, and mouseleave), the o-hoverable module must be used to make those hover capabilities configurable.

Viewport events

For viewport events that may fire several times in quick succession (scroll, resize and orientationchange) it’s good practice to throttle listeners to these. o-viewport provides pre-throttled abstractions of these events and should be used by modules that need to listen for changes to the viewport.