It's unclear why the decision was made to place the template into a separate file, but it is possible that the real use case is more intricate or there is a desire to reuse the template across various components.
Before delving into how to modify the example to function correctly, I would suggest familiarizing yourself with the concept of smart vs dumb components. Essentially, the current template possesses excessive knowledge as it dictates the presence of a function like validateForm to be invoked upon button click (creating a tight connection between the template and component, which contradicts the purpose of separating them into distinct files). A better approach, if aiming for decoupled elements, would involve providing a callback to execute when the button is clicked - allowing flexibility in defining the action to be taken.
To address your example's issue and enhance its adaptability, understanding the significance of 'this' in JavaScript is crucial. 'This' denotes the current context of a function, varied depending on its creation and invocation method.
In your template.ts file, there isn't a designated function (though it would function as one due to module transpilation), yet 'this' persists. In this scenario, 'this' references the module's context itself. Thus, attempting to call module.validateForm within the template snippet leads to failure since it doesn't exist - this occurs because the snippet materializes during module loading rather than when required by render.
The primary step towards resolution entails encapsulating the snippet within a function and exporting it for subsequent utilization inside render, enabling control over 'this' within the function's scope. If necessary, employing 'bind' may be vital to ensure 'this' aligns with the component class.
A complete exemplar can be found below:
Component.ts
import { LitElement, html} from 'lit-element';
import { cTemplate } from './template/ctemplate';
@customElement('card-form')
export class cardFormComponent extends LitElement {
constructor() {
super();
this.cTemplate = cTemplate.bind(this);
}
render() {
return this.cTemplate();
}
validateForm() {
alert('ok');
}
}
**template.ts**
import { html } from 'lit-element';
export function cTemplate() {
return html`
<div>
<button class="button" @click="${this.validateForm}">Validate</button>
</div>
';
}
For enhancement:
BetterComponent.ts
import { LitElement, html} from 'lit-element';
import { cTemplate } from './template/ctemplate';
@customElement('card-form')
export class cardFormComponent extends LitElement {
constructor() {
super();
this.cTemplate = cTemplate.bind(this);
}
render() {
return this.cTemplate(this.validateForm, 'Validate');
}
validateForm() {
alert('ok');
}
}
**template.ts**
import { html } from 'lit-element';
export function cTemplate(onClick, label) {
return html`
<div>
<button class="button" @click="${onClick}">${label}</button>
</div>
';
}
This pattern appears somewhat unconventional. If the intention is to create a reusable button component, consider developing a standalone web component for it. Within the cardForm, utilize render to generate something akin to:
html`<my-button label="${'Validate'}" .onClick="${this.validateForm}"></my-button>`
You could also configure my-button to emit a click event similar to a standard button and leverage @click accordingly.