Communicate from Parent to Child Component in LWC

There are few scenarios where we need to pass property/values from parent component to child component in LWC. There are different ways to achieve this:

• Using @api decorator (public property)

The @api decorator in the child component makes property as public, which can be utilised by parent component to update it. So whenever you add child component in parent component, then specify child public property as an attribute and pass the value from parent component property. 

For example, Child component contains public property "passedUrl" with @api decorator

-----------childComponent.html--------------------

<template>
    <div style="background-color:#b2b689;border-style: solid;">
        <b>Child component</b>
	<p>URL passed from parent-<b>{passedUrl}</b></p>
    </div>
</template>

--------childComponent.js---------------------

import { LightningElement,api} from 'lwc';
export default class SkChildComponent extends LightningElement {
    @api passedUrl;    
}

Now if you have to pass value to "passedUrl" property from parent component then just specify attribute as property name and pass any value from parent component property.

--------parentComponent.html---------------------

<template>
    <lightning-card  title="Communicate from Parent to Child">
        <div style="background-color:#E6E6FA;border-style: ">
            <b>This is parent Component</b>            
            <c-child-component passed-url={selResourceURL} >
            </c-child-component> 
        </div>
	</lightning-card> 
</template>

--------parentComponent.js---------------------

import { LightningElement,track } from 'lwc';
export default class SkParentContainerCmp extends LightningElement {
	@track selResourceURL='https://www.sfdcstuff.com/';
}

• Using getter setter property in child component

you can define @api property with separate getter setter defined. Whenever the value of this property changes from parent component, your setter method for that property will always get called. Now you can define your custom logic if you have and then utilise that in child component.

--------childComponent.js---------------------

import { LightningElement,api,track } from 'lwc';
export default class SkChildComponent extends LightningElement {
    @track _userInputNumber;
    @api
    get userInputNumber(){
        return this._userInputNumber;
    }
    set userInputNumber(value){
        //write your logic to set any property
        this._userInputNumber=value;
    }
}

• By accessing child function from parent component

You can decorate the function defined in child component with @api. Now parent component can access this public function. By using this method, you can invoke function defined in child component.

Suppose you have defined below function in child component:

@api
increaseCounter(){
   //your logic
}

Now from parent component you can access this function by using below syntax:

this.template.querySelector('c-child-component').increaseCounter();

I have created parent and child component to explain how this communication from parent to child component happen. Please find below complete code snippet and components snapshot:

Complete Code

Hope this will Help!!