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Mavo Internals: An Overview

This documentation describes the purpose or function of essential classes and properties in the Mavo codebase (inside src) to help facilitate your contribution process. It's a work in progress and feel free to suggest edits to this documentation.

Accessing Mavo properties in the browser console

  • Mavo.all lists all Mavo instances.
    • Indexed by the ID of the app (mv-app attribute).
    • Can also be accessed with their index (e.g. [0], [1]).
  • _ is an alias for the current Class defined in a source file.
    • Mavo.all ( _.all ) is static, thus is able to get all the Mavo instances when called.
  • Can also use Mavo.all.[app-name].children to access properties.


This file is the entry point of Mavo and all of Mavo's components are utilized here.

mavo.js Constructor

  • treeBuilt Promise is resolved when the Mavo tree is created.
    • It's a structure that represents the data schema (built before data is fetched).
  • dataLoaded Promise is resolved when all data is loaded.
  • Mavo.Promise (util.js) is a helper that creates Promises that resolves externally.
  • inProgress is a live attribute: It doesn’t only hold a value but also executes code when it runs.
  • ._ is also a method for localization but it's always a property of something, unlike the _ that represent the current class.
    • If your code adds any text that is visible to the end user in any way, the final version should not put the text as a string in your code. Instead, a phrase should be added to the localization file and use the ._ method to retrieve it.
  • Mavo figures out if an element is a group (heuristics) but the user needs to add a typeof attribute to the element to explicitly declare a Mavo group.
    • The name typeof is used because it follows RDFa.
  • Each type of element corresponds to a config which is gotten from getConfig on Mavo.Primitive.
    • Config describes things such as which attribute should be edited by default and what should it be edited with.
  • The constructor goes over the storage attributes and creates backend objects for them.
    • There’s an observer that reacts to changes to the 3 possible storage attributes (source, storage, init), which allows dynamic storage.
    • Every Mavo app's storage location can be changed through the URL.
  • this.unsavedChanges is set to false first.
    • Each node also has an unsavedChanges flag.
  • this.persmissions.onchange monitors permissions and react to any changes to permissions and reflects them on the mv-permissions attribute.
    • Mavo CSS does things (e.g. hide elements) based on the permissions.
  • if(this.needsEdit) block reacts to changes to edit, add, and delete permissions.
    • The first callback only runs when the user can at least edit, add, or delete. It observes the entire tree for changes to the mv-mode attribute(mv-mode might be redesigned in the future).
    • The block observes changes to the mv-mode attribute, figures out which node corresponds to the element that changed and puts the element in edit mode if needed.
    • The second callback executes if none of the 3 permissions are true and destroys the observer.
  • If there is a storage or source, then data is fetched by calling the load() method of the Mavo instances.
    • If no storage or source then make do with whatever is in the HTML.
  • Dynamic IDs code block observes changes to IDs and makes sure to scroll to the right element.
    • If an ID is changed after a website loads the browser won’t scroll natively.
  • Configures autosave with the if(this.autoSave) block.
  • Adds keyboard navigation with this.element.addEventListener.
  • Constructor ends with a hook (init-end).

mavo.js Properties

  • this.root.editing is a shortcut to see if the app is been edited.
  • getData() is a shortcut to the getData method of the root.
  • toJSON() gives a JSON representation of the Mavo element.
    • Can always get data this way if a bug messes up displayed data.
  • message() displays a message using the UI.Message class.
  • error() displays an error message and optionally logs in the console.
  • render() is called every time data is rendered on the entire Mavo.
    • Mavo expressions are disabled before rendering. After rendering they are enabled again and updated unconditionally.
    • Every time a value changes, expressions that reference that value change/recalculate. But when rendering data to the entire Mavo, all expressions will be recalculated afterward anyway, so instead of changing nodes before rendering, which is a waste, Mavo disables then enables after rendering to update them all at once. Then during the lifetime of the Mavo app, the usual pattern remains.
  • edit() is a shortcut to editing the root element and has code that highlights the current collection item when it's hovered.
  • done() is present on all Mavo and Node instances, which concludes the edit and removes any event with a class of mv-edit and sets unsavedChanges to false.
  • setUnsavedChanges() is a helper method that sets the unsavedChanges flag on all child nodes.
  • updateBackend() reads the URL parameter in case the user is overwriting the backend with a URL parameter.
    • Allow one to use own data on other Mavo apps.
  • load() is the method that calls render() and checks which backend to load from (do we have storage, source, or init). When the backend is ready, then it tries the load() method of the backend object and if there’s an error then it tries again with init if there is mv-init. If there’s no init then it propagates the error with Promise.reject(err). After this, errors are actually handled and error messages are displayed. After data is loaded from the backend, render() is called.
  • store() stores data, sets inProgress to saving, calls getData to get data to store, then calls the store() method of storage backend. After this the storage backend any details.
  • upload() is called when want to upload a file. Primarily used now by the image editing widget. Eventually, there will be other properties with upload().
  • save() calls store(). After things are saved, it calls the save() method of the root node, which propagates it down the tree and calls the save method() of every children node. It then sets unsavedChanges to false.
    • save() on nodes take care of the UI that needs to change after data is persisted.
  • changed() is called when there’s a change in the tree and takes care of updating expressions.
  • setDeleted() is called when nodes are deleted. Takes care of the UI to let users undo, and to also delete permanently soft-deleted items
    • Remove items from collections, but keep them in the deleted array.
  • All properties under live: {} executes extra code when they are set or get.
  • Can have multiple permission objects with the same parent and there are certain rules about how these permissions combine on the actual Mavo instance.
  • get() gets the Mavo instance that corresponds to an element.
  • attributes object is an array of all attributes. It is incomplete as more are in separate JS files and are pushed later to this object.

How Mavo Picks Up mv-app Elements

  1. Fetches polyfills needed (; syntax-level features cannot be polyfilled.
    • Mavo has a list (IntersectionObserver, Symbol) that needs to be polyfilled.
  2. Adds/pushes dependencies after the polyfills are loaded.
  3. Loads Mavo plugins using _.Plugins.load() defined in plugins.js.
  4. _.inited Promise resolves after page ($.ready) and dependencies are ready.
  5. _.init() gets all elements with an mv-app attribute that matches the $$ selector (using selectors object).
    1. Filters array of mv-app elements (use Mavo.get to see if there’s another mv-app associated with this element).
    2. Maps each element to a new Mavo instance & return this array.


This file has utility functions used throughout the codebase. All functions are added to the Mavo object.

  • load() loads CSS or JS files asynchronously and returns a promise.
  • readFile() reads and returns the file content.
  • toJSON() converts to JSON with a given format.
  • saveToJSON() convert to JSON even if you have loops.
    • When an object has a circular reference, this function will drop the reference.
  • objectify() takes a primitive and returns an object that functions as closely as possible to the primitive.
    • Allows Mavo primitives to have metadata/properties (e.g. toNode) to make them work in Mavo expressions.
    • value() gives the primitive value w/o extra properties of Mavo primitives.
  • pushUnique() pushes an element to an array only if it doesn’t exist.
    • Most cases one should use a Set for this but can use this function if necessary.
  • filter() filters an array in place (native filter() returns an new array).
  • is() is a shortcut to check if something matches a given selector in Mavo.selectors.
  • data() gets and sets metadata associated to an element.
    • Directly adding properties to an element is bad practice.
    • Use a separate WeakMap to associate data with elements.
    • Map and WeakMap are structures that can associate any data with an object. The keys here can be anything.
    • WeakMap is good for caches because it doesn’t contribute to the references; if nothing else except the WeakMap references an object, it can still be garbage collected.
  • elementPath() returns a path from an element to an ancestor as an array. If an element is passed the method returns the path and vice versa.
    • Used in collections and Mavo expressions.
  • revocably is an object with helpers that add/remove DOM elements and still remember the place in the DOM, even if other elements have been added in the meantime.
  • inView is an object that lets you execute code when something is in view in the viewport using an intersectionObserver.
    • resizeObserver is used for the Mavo bar.


  • uid is a unique ID for all nodes in a Mavo object. It's used for debugging.
  • nodeType is a string based on whether the node is a collection, implicit collection, group, or primitive.
    • Implicit collections are created when there are two or more elements with the same property but don't have the mv-multiple attribute. They are stored like collections but cannot be edited.
    • this.nodeType = this.nodeType because they need to be set ASAP so that this property is displayed in the console when a node is logged.
  • The options passed to the constructor are then added, and they are attached to the current instance.
  • Almost every object, including Node, has a reference to the Mavo instance that created it.
  • is the group the node belongs to, which could be itself if the node is a group.
  • this.parent is the parent of the node which could be a group or collection.
    • parentGroup is whatever group contains the node. They are initialized by but its value is set later.
  • alias is the element that has the mv-alias attribute.
    • mv-alias is useful when a Mavo user wants to dynamically render data to a different node with mv-value.
  • template is the archetype of a node. Collections have a lot of copies of the same node (first node of the collection).
    • template is one of the options that is set by the constructor options object. Every time a node is created, options contains the template.
    • The archetype of each node contains a list of copies; if not a collection copies array is empty.
  • Gets the property, type, storage, and path attribute and initiate corresponding JS properties.
  • If a node belongs to a collection directly (a collection item), this.collection will point to that collection. The indirect items have a closest collection. If there’s a template and the template has expressions, Mavo doesn't traverse the DOM tree again.
  • DOMExpression object calls an expression object that is shared across all copies but the DOMExpression object is different for each expression. DOMExpression objects hold a position in the DOM and the expression object and associate the two.
  • getLiveData() gets the data used in expressions and never changes. Even if the data changes on the object, the reference to the object doesn’t change.
  • walk() helps run code on this node and every node inside it.
  • walkUp() walk the node’s ancestors starting with its parent.
  • edit() edits the element.
  • done() exits edit mode.
  • propagate() can define a method that propagates to its children recursively.
  • render() is the method called every time data is rendered on a node, either because the remote data is loaded or mv-value has a new value to render.
    • Ultimately, render() calls the dataRender() method of the node superclass.
    • The method has rules on what to do when the data of a node is unexpected.
  • dataChanged() is called every time data changes.


  • Mavo.Group constructor goes over Mavo properties inside the root element and creates corresponding objects for them.


  • Every backend extends the Mavo.Backend class.
    • Contains things that are shared across all backends (e.g. helpers).
    • Each backend class implements things that are different across backends.
    • If any method is not useful to a backend that is extending Mavo.Backend, then that backend can overwrite the method.
  • Contains helpers for loading and storing.
  • Has stubs for ready, login(), logout(), and put() to be overwritten.
    • Defined to point to something of the right type so that third-party code doesn’t break.
  • request() is a helper for making OAuth requests with JSON based APIs.
    • Most of the time only apiDomain needs to be defined to direct API calls to that domain URL.
  • For many APIs the methods of Mavo.Backend are sufficient.
  • The passive parameter in oAuthenticate() is true when you want to log a user in if there are already credentials stored and you don’t want to show any login UIs, and is false when the user has clicked login thus it’s ok to show the UI.
    • Useful when the app loads and you don’t want to show a popup before the user clicks login.
  • create() is a factory method that checks what type of backend needs to be created based on the string URL and returns the right type.
  • register() is a helper method that registers new backends.

Below are default backends.

  • HTML elements backend stores data to an element (useful for debugging).
    • e.g. Provide hash data and give an element an ID of data then all data is stored to and read from that element.
  • Remote URL backend loads data to a remote URL (doesn’t support saving).
  • localStorage backend saves data into local storage (no login).

Example: backend.dropbox.js

  • Has a unique id.
  • Sets the base permissions to login and read.
  • Logs in passively.
  • Contains a helper method to change the share URL the user provides to something different to make it CORS enabled.
  • Dropbox supports client-side authentication but it would mean that everyone who uses the Dropbox API would need to register a Dropbox app.
    • Solution: Registered own Dropbox app and the authentification is done on our server.
  • At the minimum, a new backend needs to implement put(), aka. how to save data.
  • getUser() sends a request to get info about the current user, then fires a login event, which other parts of Mavo catch and update the Mavo bar.
  • test() takes an URL or any string and returns true if this string means that an object of this type needs to be created and false otherwise.
    • Backend.create() method runs over all the backend, tries the test method for all, and the first one that returns true is the one to create.
    • The Dropbox backend test() method tests if the URL passed in is a Dropbox URL. It creates a URL based on the string and tests if the host contains Dropbox.

Example: backend.github.js

  • It has more code to handle pull request flow.
  • get() is implemented because though we could send a simple get request, we need to login to avoid the rate limit if we can.
  • put() implements saving.
    • The first parameter gets the serialized data, data as a string (usually JSON string). When it reaches the backend, it will already be in the right format.


  • Contains config info of all types of elements.
  • attribute sets the default attribute of a type of element.
  • Selector: What type(s) of element does this apply to.
  • Default: If the object is about the default attribute or not.
  • Editor creates editing interface for them.
  • changeEvents lets developers specify custom events.


  • Each plugin should have a Mavo.Plugin.register call that registers an ID for the plugin and provides an object with properties.
    • init() to initialize the plugin.
    • Hook to run code at different hooks.
    • helper methods.
  • Can’t use mv-plugin if it is not in the plugin registry. Use a <script> element to link the HTML to your plugin JS.

Miscellaneous Information

  • Collections and groups in Mavo are all objects.
  • A group's children are properties and a collection's children are array items.
  • Third-party developers can run code in hooks so this inside their code will still refer to the same instance as where the hook is.
  • Some elements need to have HTML inside it but are not groups, such as rich text editing.
  • MutationObserver watches for changes in the HTML, which allows us to react to changes in the HTML without caring what caused the change.
  • Every node has a data property that contains the raw data.
  • Lazy properties are replaced with the value their function returns when they are first referenced, and after that, they are just normal properties.
  • Symbols are used to create hidden properties on objects that no one has access to from the outside unless they have the reference.
    • Data gotten from expressions have hidden properties (symbols) like toNode and toProxy, which can be used to retrieve the node that generated these data.