Custom vs Script CPart - The general rule of thumb is that Script tags are for custom, component-specific behavior, or to fill in gaps in stitching together other CParts, while writing Custom CParts is for interfacing with async, APIs, or any time that a Script CPart gets too cumbersome. Modulo's philosophy is to allow separation of work between component development (HTML, CSS, templating, and high-level behavior), and CPart development (complex JavaScript and APIs).

Script CParts allow you to define custom behavior for components using the full power of JavaScript. The most common use of Script tags is to add more sophisticated interactive behavior. Instead of just relying on premade CParts, with a Script tag you can program any custom behavior into your component.


Script is traditionally included in Component definitions below the State or Template tag. Script has no expected attributes, but instead only has a text-content.

Typical use

The most common use of a Script CPart is to specify custom JS code that is either run once after fulling loading all dependencies, or once each time the component renders. See below for a simple example:

<Component name="ButtonExample"> <Template>...</Template> <State>...</State> <Script> console.log("Will execute once after loading dependencies"); function initializedCallback() { console.log("Executes every time an instance of ButtonExample is created"); } function updateCallback() { console.log("Executes after every rerender of the component"); } </Script> </Component>

Using within embedded component definitions

Problems arise when attempting to include a Script CPart within an embedded Component <script Modulo> definition, e.g. one that is being defined within the script tag of the same HTML file that it is being used on. This is because the < /Script> tag will end up closing off the outer <script> tag early, due to HTML syntax. The reason is that </ script> is a special syntax that automatically closes the outer tag, and also HTML cannot distinguish capital (the <Script> CPart) vs lowercase (the outer <script> tag). The ideal solution is to simply move the component to a separate library file, included with -src= syntax, since that is preferred in general, and solves other issues as well (e.g. templating instructions in HTML tags). However, if you simply must include a Script tag in an embedded component definition you can write <def Script> as the opening tag, and </def> as the closing tag. See the following:

<script Modulo src=""> <Component name="ButtonExample"> <def Script> console.log("Hello, world!"); </def> </Component> </script>

A better alternative is to use the slightly more verbose <template Modulo> instead:

<template Modulo> <Component name="ButtonExample"> <Script> console.log("Hello, world!"); </Script> </Component> </template> <script src=""></script>

Defining event callbacks

The most common purpose of a Script CPart is to add custom behavior when certain "events" occur to your component. Consider the following example of 3 click events:

<Template> <button @click:=script.doConsole>Console Log</button> <button @click:=script.doAlert>Show Alert</button> <button @click:=script.countUp>Counter {{ state.num }}</button> </Template> <State num:=42 ></State> <Script> function doConsole() { console.log("Event callback. State.num is:", state.num); } function doAlert() { alert("Event callback. State.num is: " + state.num); } function countUp() { state.num++; } </Script>

In this, the Script CPart defines a function named countUp. The @click:= attribute on the button utilizes directives to attach a "click event" to the button, such that when a user clicks on that button it will invoke the countUp function.

From within event callbacks, the Script CPart exposes the current renderObj as variables. So, state by itself is equivalent to renderObj.state. This enables us to directly modify the state by simply doing state.count++. By default, components rerender after every event, so after the event the component will rerender and visually reflect the state changes.

This means that all renderObj variables will be available here, in a similar way to how Template exposes them: For example, you can use code like props.XYZ to access data from a Props CPart.

You can also access the JavaScript Object instances of the CPart Class. To access those, you use the cparts

Finally, the variable element will refer to the HTML element of the current component. This is useful for direct DOM manipulation or interfacing with other frameworks or "vanilla" JavaScript. Generally, however, you should avoid direct DOM manipulation whenever you can, instead using a Template CPart to render the component (otherwise, the Template will override whatever direct manipulation you do!).

Template vs renderCallback Before replacing a Template with a renderCallback, first consider using prepareCallback or extending the templating language with a custom filter. The Templating language is intentionally limited and result in more readable code and a cleaner separation of concerns of logic (JavaScript) from presentation (HTML templates).

Defining lifecycle callbacks

By naming functions with certain names, you can "hook into" the component rendering lifecycle with callbacks.

You can define a function with any of the following names and expect it to be invoked during it's namesake lifecycle phase: initializedCallback, prepareCallback, renderCallback, reconcileCallback, and finally updateCallback.

See below for an example of defining a custom prepareCallback in order to "hook into" the component rendering lifecycle to execute custom code. The return value of this function is available to the Template CPart when it renders during the render lifecycle.

<Template> {% for item in %}<p>{{ item }}</p>{% endfor %} <p>(C) {{ script.year }} All Rights Reserved</p> </Template> <Script> function prepareCallback() { return { data: ["a", "b", "c"], year: (new Date()).getFullYear(), }; } </Script>

Template literals? Modern JavaScript has a built-in mini-templating system. Learn more at MDN's Template literals reference.

For fine-grained control over a component's rendered HTML, you can hook into the renderCallback. While not recommended for most usage, it remains an option to use the renderCallback to write JS that constructs the HTML in a custom fashion, such as using a third party templating system, or simply using template literals.

Another option is to use this to, for example, use backtick template literals for templating, if for whatever reason Modulo's templating language is insufficient. See below:

<State name="Ha-Joon Chang" books:='[ "Edible Economics", "Kicking Away the Ladder" ]' ></State> <Script> function renderCallback() { component.innerHTML = ` <p>Author: ${ }</p> <ol>${ => ` <li>${ title }</li> `).join('') }</ol> `; } </Script>

Defining directive callbacks

You can also create "custom directives" from within the Script tag. Read more about this in the section on Directives

Execution context

Static execution context (default)

By default, the factory phase is when the code in the script tag itself gets executed. That is, it is executed exactly once while the component class is being prepared, before anything is yet even mounted on the document. This means that that any loose variables defined will be "static", or shared between all instances of that component.

Static execution Script tags will only correctly isolate synchronous code. It is thus recommended to keep your Script tag to only contain synchronous code, that is, no async callbacks. This is an intentional limitation, as Script tags are intended for synchronously patching together behavior around potentially asynchronous CParts. There is also a potential for a bug: If there are multiple instances of the same component on the same page, with async callbacks ready at the same time, it can cause the wrong state, props or other variables or CParts to get mixed up and go to the wrong component. Asynchronous coding should be thus moved to a custom CPart, that is then used in the script tag in a declarative, synchronous manner. The CPart API gives you low-level access, and makes no assumptions about using a synchronous or asynchronous coding style.

That said, Modulo is intended to be useful as a glue language and it's common to find asynchronous snippets that you might want to quickly integrate into an existing component. Read on for a few ways to bend this rule in order to better integrate third party code.

Lifecycle execution context

ALPHA API NOTE: This feature might be renamed in a future release of Modulo alpha: See Issue #26

Per component: lifecycle="initialized"

Setting lifecycle="initialized" behaves very similarly to the "static" execution context (the default): For both, functions defined there will be available to be attached to events, and you can define Callback and Mount and Unmount functions. However, there is one important difference: It will execute each and every time an instance of the component first mounts on the page. That is, it will run everything in that script tag once per component use. This keeps all the data in the script tag private to that component, allowing for asynchronous code.

It also allows use of the private variables as a sort of state. That is not recommended usage, but might be useful for integrating with other JavaScript state or store management systems.


The Script CPart "exports" a variety of properties to the renderObj, all of which should be considered "read-only".

  • script.someFunctionName for functions declared like function someFunctionName - In addition to "exports", the script tag will also make available all functions that you have declared using the "old-school" named-function style syntax. This is what allows attaching events to be so simple: Simply define a named-function in your script tag (e.g. function doStuff(payload, ev) { ...), and then it can be referenced elsewhere (e.g. in the template with: @click:=script.doStuff template with a click event like: It will ignore any "arrow functions" (() => {}), or any other anonymous functions (e.g. functions declared like const myFunc = function () { ... will also be ignored). That's not to say you can't declare your functions like this: However, you should only do so if you do not want them to be automatically exported.