React/ReactJS: Passing Props (w/ Methods) to a Functional Child Component

In React's unidirectional flow of data, props are the easiest mode to pass data, state and methods from a parent component to a child, and they are read-only.

In the example below, the parent component <Deer/> passes a props (an object) consisting of the JSX attribute called skinColour with the value brown to its child component <Fawn/>. 

				
					// Parent Component
					class Deer extends React.Component {
					  render () {
					    return(
					      <Fawn skinColour={'brown'} />
					    )
					  }
					}
A Doe & Fawn by Marshal Hedin from San Diego. CC BY-SA 2.0. Used in AI vocal remover.

Inside the child class component <Fawn/>, the property skinColour can be accessed via this.props as follows: 

				
					// Child Class Component
					class Fawn extends React.Component {
					  render () {
					    return(
					      <div style={{'color': this.props.skinColour}}>
					      	I AM THE SKIN COLOUR OF A FAWN
					      </div>
					    )
					  }
					}

By the way, if there are not going to be any states in a component as in the above case (only render()), it is a better approach to opt for a functional component instead of a class component.

Passing Props to a Functional Child Component

But unlike class components (which are ES6 classes), functional components cannot have constructor() or naturally inherit props from the parent components. You have to pass props as an argument (that is what it is originally) to the functional component. And we do not use the this keyword before props. Also note the absence of the render() method. 

					
						// Child Functional Component
						const Fawn = (props) => {						
						    return(
						      <div style={{'color': props.skinColour}}>
						      	I AM THE SKIN COLOR OF A FAWN
						      </div>
						    )						 
						}

Passing Method to a Functional Child Component

A method is also passed to a child component as any other JSX attribute-value pair. And as in plain JavaScript, we do not attach any parenthesis () while passing methods as arguments. Below we define a method called updateColour() in the parent component <Deer/>. We pass it as the attribute updateSkinColour, with its value referencing the updateColour() method with the this keyword. 

					
						// Parent Component
						class Deer extends React.Component {
						  updateColour() {
						  	console.log('the colour is biege');
						  } 
						  render () {
						    return(
						      <Fawn skinColour={'brown'} updateSkinColour={this.updateColour} />
						    )
						  }
						}

Inside the child component <Fawn/>, we access the passed method as props.updateSkinColour. We use it as a callback function to the click event of a <button/> element. 

										
						const Fawn = (props) => {
						    return(
						      <section>
							      <div style={{'color': props.noseColour}}>
							      	I AM THE NOSE COLOR OF FAWN
							      </div>
							      <button onClick={props.updateSkinColour}>
							      	Update Colour
							      </button>
						      </section>
						    )
						}

On click of it, the event gets triggered and the callback function prints the following message in the browser's console. 

										
						the colour is biege

We can implement it using the state variable to update to another colour dynamically on click, thereby replacing the initial attribute-value pair skinColour={'brown'} with skinColour={this.state.skinColour}. 

					
						// Parent Component
						class Deer extends React.Component {
						  constructor(props) {
						  	super(props);
						  	this.state = {
						  		skinColour: 'brown'
						  	}
						  	this.updateColour = this.updateColour.bind(this);
						  }
						  updateColour() {
						  	this.setState({
						  		skinColour: 'burlywood'
						  	});
						  } 
						  render () {
						    return(
						      <Fawn skinColour={this.state.skinColour} 
						      				updateSkinColour={this.updateColour} />
						    )
						  }
						}