Low-level interface - Component developers will typically only need to use the name="..." attribute on Component, and sometimes mode="vanish-into-document" for page-level components. Similarly, none of the renderObj.component properties are typically useful for component developers. Instead, they are used internally by CParts (notably Template) in order to move along the rendering cycle.

The Component definition is the most central to Modulo. It's how we register components to be mounted on the page, and define the CParts that go inside those Components. It's also what stitches together the "machinery" that makes reconciliation and rendering even possible. In other words, the Component is what handles the "low-level" operations of mounting, reconciling to generate patches, and applying those patches to rerender, and the interface described here allows for low-level manipulation of this process for unusual circumstances when it's needed.


The Component reads several properties from the renderObj, at different lifecycle phases. Read below on how to interface with it.

  • component.originalHTML - Read-only - This contains the initial HTML that the element had when mounted on the page.

  • component.innerHTML - Write-only - Assign to this to cause the Component to attempt to reconcile the current HTML with the target HTML provided. It functions a little bit like HTML's built-in "element.innerHTML": Assign a string containing HTML code to this property to see the HTML appear on the page. This is what the Template does behind the scenes. Unlike HTML's built-in "element.innerHTML", assigning to this renderObj property will not cause an full update, overwriting everything that exists, but instead will cause the Component to employ the "reconciler" specified to generate a patches list (see next). By default, it will then also apply those patches in the update lifecycle.

  • component.patches - Between the reconcile and update stages, the "patches" property is exposed to allow access to or modification of the patches that the Component intends to apply. This is an advanced feature that gives fine-grained control to how rendering works, and thus is only useful in rare situations where that control is needed. For example, you could use this property to "intercept" the patches that are about to be applied to your component while re-rendering, and, for an example, forbid the removal of attributes by deleting all patches with "removeAttribute". This is in the format of an Array. Each item in the Array is itself a 4-Array (Array of length 4), in the following format: [element, method, arg1, arg2]. For example, to do a "removeAttribute" patch on an "id" attribute for a given "target element" (a real DOM node on the page somewhere), it might be [(ref to "target element"), "removeAttribute", "id", undefined] If the Array is empty, then no patches will be applied.

Examples of patches

Using console.table(component.patches), you can easily inspect the exact order of DOM operations that Modulo will do for every operation. This produces useful results in both Firefox and Chromium-based browsers.

<Template> <label><input [state.bind] name="enabled" type="checkbox" /> Show patches in Developer Console</label> <br /> <hr /> <h3>To Do example:</h3> <ol>{% for item in state.list %} <li>{{ item }}</li> {% endfor %}</ol> <p> <input [state.bind] name="text" /> <button @click:=state.list.push payload="{{ state.text }}">+</button> </p> </Template> <State num:=42 enabled:=false list:='["Milk", "Bread", "Candy"]' text="Coffee" ></State> <Script> function updateCallback() { if (state.enabled) { console.table(component.patches); } } function countUp() { state.num++; } </Script>


Components use their attributes to set several configuration options. They take the following attributes:


name - REQUIRED - The "name" of the component. Conventionally, it should be written in camel or dromedary case, like how class names are written in JavaScript: LikeThis. Technically, it's case insensitive, so a component named like LikeThis, imported with an x- namespace, could also be used like x-likethis or X-LIKETHIS.


rerender - default is rerender="event". Specify mode to change the render mode of this component. A detailed discussion of valid options are below:

  • rerender="event" - The default behavior, where the the component will rerender after ever event it handles (e.g. after every user interaction). This is typically desirable since usually every user interaction will cause some visual change that needs to be reflected.
  • rerender="manual" - Modulo will not auto-rerender in any situation. This Note that if this is set, you will have to manually rerender, such as in a Script CPart (element.rerender()), or in a custom CPart (this.element.rerender()) is invoked. This is useful if there are many events that don't cause changes (e.g. mouse movement), but you have discovered that the extra rerender invocations are impacting performance, even though they aren't generating patches to modify the DOM.


mode - default is mode="regular". Specify mode to change the DOM root render mode of this component. This changes what is considered to be the root of the element, and thus where the content of the Template goes during reconciliation. In "regular" and "vanish", it's the custom component itself (which will be removed after rendering with "vanish"), in "shadow", it uses an attached shadow DOM as the root, and with "vanish-into-document" it replaces the entire page. This allows you to isolate outside CSS from your component using "shadow", do one-time renders only with "vanish", or make your component replace the entire document with "vanish-into-document". A detailed discussion of valid options are below:

  • mode="regular" - The default behavior, where the content generated by this the element will be attached to the regular DOM, as the element itself. This means that CSS stylesheets attached the normal way (e.g. with a "link" tag) will affect the contents of this component. Style CParts are, however, automatically scoped to the component, so a selector like p {...} will get prefixed like x-MyComp {...} when CSS files are being generated from Style CParts. Keep in mind the normal CSS rules will still apply, meaning that auto-scoped "p" tag will also affect children, grandchildren, etc of sub-components (unless those are "shadow" based).

  • mode="shadow" - Use the so-called "shadow DOM" to render the content generated by this component. While the shadow DOM may sound like a sci-fi villain, it's a mechanism browsers provide for custom elements to isolate their contents from CSS selectors or other JavaScript libraries. This isolation means means that CSS stylesheets attached the normal way (e.g. with a "link" tag) will not affect the contents of this element. Note that other than the isolation provided, the shadow mode should work exactly the same as regular. That said, Style CParts will still be automatically scoped to the component, but will use the shadowDOM feature to more strictly enforce that scoping. Note that shadowDOM CSS will not get included in the CSS file bundle, but instead will be bundled in the JavaScript source code. For a component by itself, switching from "shadow" to "regular" or vice-versa shouldn't change behavior at all. It's only useful when CSS from one component or a third party CSS library is "getting inside" your other components in an undesirable way, and you want to isolate them.

  • mode="vanish" - This less-used setting causes the component to "vanish" after rendering, or replace itself with it's children. Setting this will cause the component to remove itself after the first time it renders, effectively using the component as a simple template. All CParts will become useless after the first rendering, so CParts like Script or State are rarely useful in combination with this mode. This "one-time render" feature is most useful for static pages when you don't want your custom components to get in the way of the DOM that is generated, and for creating static-site generators, when you just want to generate plain HTML, with no Modulo JS or behavior in the end.

  • mode="vanish-into-document" - This setting is useful in one situation: When you want to create a "page" level component that changes tags that belong in the document head, such as <title>. Like with mode="vanish" described above, setting this will cause the component to remove itself after the first time it renders. However, with vanish-into-document, it will instead replace the entire page. It will also attempt to correctly insert all tags that belong in the document head (meta, title, link, script to be specific), causing link and script tags alike (e.g. <script src=".."></script>) to load. Finally, the document will be wiped, and anything else it finds will be put directly into the document's body, for a clean DOM structure that removes itself entirely during this "one-time render". * DEPRECATION NOTICE - vanish-into-document may be removed, and simply vanish will also support insertion to head

  • mode="custom-root" - This is rarely useful, but allows for setting some other DOM element as the new root element for the component to target for rendering it's content. This is done by assigning a value to renderObj.component.root before reconciliation. If no .root value is set, an error will occur.


The Component defines 2 directives:

  • [component.event] (shortcut: @) - Attach event listeners to DOM elements, and remove them when the DOM elements are removed. (For jQuery users, this is used for similar purposes as "live" (delegated) events, but is faster.)

  • [component.dataProp] (shortcut: :) - Attach data to a DOM element's .dataProp object, which can be used to directly pass renderObj values as Props or events


Reattach the original child elements that this component had when it was first mounted to a new DOM element. (For React users, this is similar to doing {this.props.children}.) For more on the slot interface, see Mozilla's The Web Component Slot element

Note that Modulo uses this interface for both mode="shadow" (e.g. using shadowRoot as the root element for reconciliation) and all the other DOM rendering modes (e.g. mode="regular", mode="vanish-into-document"). For Shadow DOM, it will fall-back on Browser behavior. For everything else, it will detach any nodes it finds and reattach the nodes just before the beginning of the reconciliation process (it's DOM load-directive).


The HTML content of the slot will be untouched if no elements are provided. This allows you to use this like a default or placeholder. View the USAGE mode to see how it's used.

<Template> <p>Hey <slot>First Name</slot>, what's up?</p> </Template>
1: 2: Richarlison

Named slots

If you give a name to your slot (e.g. <slot name="content"></slot>), then it will only be attached to similarly "slotted" elements (e.g. <div slot="content">...</div>). Below we have an example showing off multiple named slots. View the USAGE mode to see how it's used.

<Template> <div class="fancy-frame"> <h1><slot name="topTitle">Picture</slot></h1> <slot> <em>No caption provided.</em> </slot> </div> <div class="fancy-frame"> <slot name="photograph">No photograph provided</slot> </div> </Template> <Style> :host { text-align: center; } .fancy-frame { display: inline-block; border: 10px inset salmon; padding: 10px; margin: 10px; width: 100px; background: pink; } </Style>

Caption: The Return of the Hippo