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Why do we need track decorator in LWC?

 LWC engine can track changes or value assigned to a field or primitive properties automatically, however if field/properties is array or object type then LWC will not be able track value change automatically. 

That is when @track is helpful, this will tell LWC engine to observe changes to the properties of object or elements of array decorated with @track.

Here is my component code

We  can also achieve using spread operator, the key here is instead of mutating same object/array we can create copy and make changes. LWC framework observes value assigned to field/property and if new value !=== previous value then component re-renders.

this.address = {...this.address,"city":event.target.value}

<template>
    <lightning-card title="2 way Data Binding">
        <div class="slds-m-around_medium">
            <lightning-input type="text" label="Enter course Name"
                onkeyup={changehandler}></lightning-input>
            <div>{fullname} is course of {title}</div>            
        </div>
    </lightning-card>
    <lightning-card title="@track properties">
        <div class="slds-m-around_medium">
            <lightning-input type="text" label="Enter City Name"
                onkeyup={trackhandler}></lightning-input>
            <div>{address.city} is my city</div>            
        </div>
    </lightning-card>  
</template>

Java script code

import { LightningElement,track } from 'lwc';

export default class HelloWorld1 extends LightningElement {
    fullname = "LWC Learning"
    title ="aura"
   
     @track address = {
        city:"Louisville",
        postalCode:40291,
        country:"USA"
    }
    @track hobbies = ['Music','Reading','Netflix'];
    changehandler(event){
        this.title = event.target.value;
    }
    trackhandler(event){
        //this.address = {...this.address,"city":event.target.value}
        this.address.city=event.target.value;
    }
   
}
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