📣 UPDATE! 📣
Now lit-html and LitElement are unified under Lit.
I'm writing new posts about Lit, meanwhile you can:

• read this post because the principles are the same
• upgrade your code with this guide
• visit Lit site to know what's new

You can also use lit-html standalone

Although lit-html is very efficient rendering, it is better to render only when necessary. That is why LitElement differs the re-rendering of the UI by batching the property updates. In other words, re-rendering is done asynchronously to ensure efficiency and good performance. Let's see how the update process works.

Recall from the previous post that all the properties defined in the properties getter become properties "controlled" by LitElement.

For each property defined in the properties getter, LitElement will generate:

• a hasChanged function
• a setter and a getter
• an observed attribute
• a fromAttribute function
• a toAttribute function

In particular, we are interested in the hasChanged function and the setters because they play an important role in the update life cycle.

Almost all the "magic" is based on the setter that causes the component to be re-rendered when the property changes. First, it checks if the property has changed (invoking the hasChanged function) and, if that is the case, it will make a rendering request.
Also, if the property is reflected in an attribute, the setter will update the observed attribute using the toAttribute function.

In the LitElement class we find the following methods that participate in the update of the UI:

• requestUpdate
• performUpdate
• shouldUpdate
• update
• render
• firstUpdated
• updated
• updateComplete

Now that we have seen the main pieces involved in the re-rendering, we will go into detail on how this process occurs.

The update cycle in action

Imagine you have a function with this piece of code:

const el = document.querySelector('#myElement');
el.title = 'Movements'; //previous value was 'No title'
el.icon = 'book.ico'; //previous value was undefined
await el.updateComplete;

• 1. The title property setter is executed. This setter calls hasChanged function of the title property. As it has changed, it returns true and in that case it calls performUpdate, a method of the LitElement class. This method verifies if there is a previous request pending, in that case it does nothing. If there isn't, it will create a micro-task (a promise) to execute the rendering. This is LitElement's way of asynchronously executing the render method and batch property changes.
• 2. We continue with the following line. Now the icon property setter is executed. The setter calls hasChanged function, which returns true, so it calls theperformUpdate method, but as a UI update operation is already pending, it does nothing else.
• 3. Finally, our code is awaiting for the updateComplete promise to be resolved, that will occur when the update cycle is over.
• 4. Now that there are no tasks on the stack, it is time to execute the micro task that was scheduled (in the step 1). It does the following:
• 4.1. It invokes shouldUpdate, another method of the LitElement class. This method receives the properties that have changed and their old values. The purpose of this method is to evaluate all the batched changes that have occurred and based on that decide whether or not the update should be done. By default it returns true, but LitElement gives us this hook in case we want to put a special logic to avoid the update. Following our example, shouldUpdate receives title => 'No title', icon => undefined and returns true, so the update continues.
• 4.2. It executes the update method of the LitElement class. Here the changes are reflected to the attributes to maintain synchrony between properties and attributes (only for those properties defined with reflect). Then it calls the render method.
• 4.2.1. The render function is executed, the DOM is updated.
• 4.2.2. If it is the first time the component is rendered, the firstUpdated method will be executed. It's a hook that LitElement gives us to over-write if we need to do initialization tasks once the component is rendered.
• 4.2.3. Then the updated method of the LitElement class is executed. This is another hook. Unlike firstUpdated, this method will always be called after every rendering.
• 4.2.4. The updateComplete promise get resolved.

👉 All the properties defined in the properties getter become properties "controlled" by LitElement and any update of their values ​​will cause a rendering. This is why we should put there only the properties that affect the visual representation of the component: those that we have used in the template.
For example, if we have a debug property that is used to indicate if traces are written by console.log, changing this property should not cause an UI update.

Live example

To understand it better, I have made this very silly component. The important part is that I have traced each method of the update lifecycle.

• When the component is rendered the first time you can find among the traces that there is an invocation to the firstUpdated method.

• I have traced the _requestUpdate and _enqueueUpdate methods that are private methods of UpdatingElement which is the class of which LitElement class extends. Although these methods are not an "official" part of the update lifecycle, seeing how LitElement uses them internally helps to understand the update process. We can see that _requestUpdate is called for every property that changes but _enqueueUpdate it's called only once: with the first property that changed. When there's an update process pending, _enqueueUpdate is not invoked again.

• The first time you press the button, it will update the title and icon properties. At this moment the component will be already rendered so you won't find a call to firstUpdate.

• The second time you press the button, it will update again the title and icon properties with the same values it did before. As the property values have no changes, the update cycle is not triggered.

• I have included the source code of the UpdatingElement class because I think you can understand it and see in detail how the UI update process is implemented.

requestUpdate

Sometimes it may be necessary to manually trigger the re-rendering cycle. It is not enough to invoke the render method, because as we have seen, the process is much more complex. For these cases, LitElement provides the requestUpdate method that will trigger the whole lifecycle update.

A typical case where we should invoke requestUpdate is when we define our own setter and we want that a change in the property to cause a re-rendering. As we have seen before, for each controlled property LitElement generates a setter that evaluates whether the property has changed and if so, it updates the UI. But when we write our own setter, we lose the setter that would be generated by LitElement, because of this, we have to do by ourselves what LitElement does. We should do:

set title(value) {
  if (this._title !=== value) {
    const oldValue = this._title;
    this._title = value;
    this.requestUpdate('title', oldValue); // Called from within a custom property setter
  }
}

👉 When we invoke requestUpdate from a setter we must pass the property name and the old value as arguments. On the other hand, if we force a re-render from elsewhere, requestUpdate has no arguments.

This is the end

With this last topic about the UI update lifecycle we complete the basics of LitElement. You already have the fundamental knowledge to continue your path in lit-html and LitElement.

To go deeper in these libraries I highly recommend reading the official documentation. It's It is very clear, concise and very well organised. In addition, each topic is accompanied by a live example in Stackblitz.

• Lit project guide

I also suggest you read the source code of lit-html and LitElement. It is relatively short, well documented and with what you already know you will not find it difficult to understand. This way everything will be much clearer.

• Lit source code

Last but not least, two important contributions from the community:

• awesome lit - A wonderful collection of resources made by @serhiikulykov.
• open-wc - Web component recommendations with a bunch of powerful and battle-tested setup for sharing open source web components.

Last words...

With this post I finish my series on LitElement. I hope you found it useful and enjoyed it as much as I did writing it. Thank you for having come this far! ❤️