Decorators in Lightning Web Component

In Lightning Web Components, we have three unique decorators that add functionality to a function or property. Decorators dynamically alter or modify the functionality.

What are Decorators in Lightning Web Component?

A Decorator is a design pattern that allows adding behaviors to Javascript Objects. Decorators which are part of ECMAScript are used to dynamically alter or modify the functionality.

Type of Decorators in Lightning Web Component

There are three type of Decorators in Lightning web components.

1. Api
2. Track
3. Wire

Let see them in details.

1. @api

To expose a public property or a public method, decorate with @api. Public properties are reactive, also known as public reactive properties since if a value of property changes then component is re-rendered

• Public properties define API of a component whereas public methods are part of a component’s API
• A Component is re-rendered when the value of a referenced public property is modified or changed
• To pass data from parent component to child component, @api decorator in the child component exposes a property by making it public so that parent component can update it
• @api properties can be exposed in an App builder

Please find the below code snippet that provides an insight how an @api property on child component is set from a parent component:

apiDecoratorSampleChildComponent.html

<template>
    <lightning-card title="Child Component">
        <div class="slds-p-around_medium">
            <p class="slds-p-horizontal_small">{message}</p>
        </div>
    </lightning-card>
</template>

apiDecoratorSampleChildComponent.js

import { LightningElement, api } from 'lwc';
 
export default class ApiDecoratorSampleChildComponent extends LightningElement {
    @api message;
}

apiDecoratorSampleChildComponent.js-meta.xml

apiDecoratorSampleParentComponent.html

<template>
    <c-api-decorator-sample-child-component message = 'Message From Parent Component!!'></c-api-decorator-sample-child-component>
</template>

apiDecoratorSampleParentComponent.js

import { LightningElement } from 'lwc';
 
export default class ApiDecoratorSampleParentComponent extends LightningElement {}

2. @track

To expose a private property or a private method, declare with @track. Also known as Private reactive properties

• To track a private property and to re-render component when that property changes, use @track decorator (Available only for the component where it is declared)
• Fields which are using @track decorator that contains an object or an array, tells the framework to observe changes to the properties of an object or elements of an array
• After Salesforce Spring ’20, all the fields in a Lightning Web Component are reactive. If a field is used in a template & value changes, the component re-renders and displays a new value by default

helloWorld.html

<template>
    <lightning-card title="Hello World" icon-name="custom:custom14">
      <div class="slds-m-around_medium">
        <p>Hello, {greetingMessage}!</p>
        <lightning-input label="Name" value={greeting} onchange={changeHandler}></lightning-input>
      </div>
    </lightning-card>
  </template>

helloWorld.js

import { LightningElement } from 'lwc';
export default class HelloWorld extends LightningElement {
  //@track greetingMessage = 'World';//Before Spring ’20 to need to import track decorator & use it to make a field reactive
  greetingMessage = 'World';
 
  changeHandler(event) {
    this.greetingMessage = event.target.value;
  }
}

3. @wire

• Reactive wire service is utilized in Lightning Web Components to read the Salesforce data from apex class into Lightning web components
• Component is re-rendered when wire service provisions the data from apex class. The output from apex method is set to a property

Syntax:

import <methodName> from ‘@salesforce/apex/<Namespace.ApexClassName.apexMethod>’;

@wire(methodName, {methodParams})

propertyOrFunction;

methodName: A variable that identifies the Apex method. 

apexMethod: The name of the Apex method to import. 

ApexClassName: The name of the Apex class. 

Namespace: Defines the namespace of the Salesforce organization. Only specify a namespace when the organization doesn’t use the default namespace (c)

displayContacts.html

<template>
    <lightning-card title="Contacts" icon-name="standard:contact_list">
        <div class="slds-m-around_medium">
            <template if:true={contacts.data}>
                <template for:each={contacts.data} for:item="con">
                    <p key={con.Id}>{con.Name}</p>
                </template>
            </template>
        </div>
    </lightning-card>
 </template>

displayContacts.js

import { LightningElement, wire } from 'lwc';
import getContactsList from '@salesforce/apex/ContactsService.getContacts';
 
export default class DisplayContacts extends LightningElement {
 
    @wire(getContactsList) //Wiring the Output of Apex method to a property
    contacts;
}