Communicating from child lightning web component to parent lightning web component

You need to fire CustomEvent from child component to communicate to a parent component.

Here we will go through an example of a child component that displays three car company names as a radio button. When a car company is selected in the child component we will fire a carchange event from child component with the name selected by the user. Parent component will listen to this event and display the selected name in parent component.

Example

1) Create an lwc component with name childLwc

2) Create an lwc component with name parentLwc

3) Now if you surface the parent component in lightning experience, it will look like

Explanation

In childLwc component, when value of car is changed in the radio button, it calls JavaScript method in controller sendDataToParent. This function then dispatches/fires a custom event with name carchange. For every custom event, if you are passing data, you need to pass it as a JSON object with detail attribute.

parentLwc component, has a listener added for carchange event. It is achieved by adding on+eventname attribute while refering child component in html. In our case it is oncarchange attribute added at line 5 of parentLwc component html. This basically tells "whenever a carchage event is received, call displayValueInParent method in JavaScript controller.

Finally when displayValueInParent method is called, we take value from the event.details and assigns to valueSentByChild variable in JavaScript

Things to Note

• This is a very common pattern to pass values from child component to parent component
• Make sure that you are using detail attribute while sending data
• Make sure that your event name is all small letters. ie carchange instead of carChange
• Make sure that your listener name is all small letters. ie oncarchange instead of oncarChange