Building Web Components 101 - Part 3

In Part 2, I went through the fundamentals of FAST and created a custom component using FAST Element. In this article, I will explain how to create custom attributes in FAST.

I will start with where we left off in Part 2. To access the source code in this article, you can clone the project at https://github.com/tebinraouf/learning-fast/tree/building-web-components-101-part-3

or you can use Stackblitz

How to create a custom attribute?

In Part 2, we create a custom <tebin-h1> component where we hard coded the heading colour. In this article, we will create a custom attribute so that we can define the custom attribute value when the component is used.

Code:

<tebin-h1>Albert Einstein</tebin-h1>

Result:

By default, the title is brown since we defined it.

//h1.style.ts
import { css } from "@microsoft/fast-element";

export const styles = css`
    :host {
        display: block;
    }
    h1 {
        color: brown
    }
`;

We want to create a color attribute so that we can set it when the component is used like below:

<tebin-h1 color="yellow">Albert Einstein</tebin-h1>
<tebin-h1 color="#41aea9">Albert Einstein</tebin-h1>
<tebin-h1 color="blue">Albert Einstein</tebin-h1>

How to create custom attributes in FAST?

FAST offers @attr decorator to create attributes. Let's go through the steps required to use @attr decorator:

• Create a property on the custom component class. In this case, the class for tebin-h1 component is defined in h1.ts.

import { FASTElement, attr } from "@microsoft/fast-element";
import { styles } from "./h1.style.js";
import { template } from "./h1.template.js";


export class H1Component extends FASTElement {
+    @attr color: string = 'brown'
}

export const h1component = H1Component.compose({
    name: "tebin-h1",
    template,
    styles,
});

By default the value is set to brown.

• The next step is to update the template so that the value is set to the h1 tag. We are going to use inline style for this demo. However, you would normally use the the css tag or scss to style your components.

h1.template.ts now looks like the following:

import { html } from "@microsoft/fast-element";

export const template = html`
-    <h1>
+    <h1 style="color: ${x => x.color}">
        <slot></slot>
    </h1>
`;

FAST uses MVVM - Model-View-ViewModel pattern. Since we defined the color property on the class, which is the Model, then we can bind it on the view by using ${x => x.color}.

• Lastly, we will delete the style that we defined in the h1.style.ts file.

import { css } from "@microsoft/fast-element";

export const styles = css`
    :host {
        display: block;
    }
-    h1 {
-        color: brown
-    }
`;

With only a few changes, our custom element now has a custom attribute.

Markup:

    <tebin-h1 color="yellow">Albert Einstein</tebin-h1>
    <p>Life is like riding a bicycle. To keep your balance, you must keep moving.</p>

    <tebin-h1 color="#41aea9">Albert Einstein</tebin-h1>
    <p>Life is like riding a bicycle. To keep your balance, you must keep moving.</p>

    <tebin-h1 color="blue">Albert Einstein</tebin-h1>
    <p>Life is like riding a bicycle. To keep your balance, you must keep moving.</p>

Result:

In the next article, we will talk about how to bind events in FAST.