What is a React Ref?

A reference to value, function, DOM or React Element

About

A ref is a state object that holds a value in its current property for the entire lifetime of the component.

It's used to hold information that isn’t used for rendering.
Unlike with state, updating a ref does not re-render your component.
It's the equivalent of a field in class component for functional components.

Example of values

A timeout ID
A HTML DOM node element. React will put a ref property in every element that holds the HTML DOM node element.

Example with Reference on a Dom element

An class component example that will show you how to read the value of an input element.

class NameForm extends React.Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this);
    this.input = React.createRef();
    console.log("At its creation, the ref.current value is "+this.input.current);
  }

  handleClick(event) {
    console.log('The ref gives access to the HTML element via current: ' + this.input.current.__proto__.toString());
    console.log('where you have access to all HTML attribute such as the value');
    console.log('Example: the value submitted was: ' + this.input.current.value);
    event.preventDefault();
  }

  render() {
    return (
      <div>
        <label>
          Name:
          <input type="text" ref={this.input} defaultValue="ChangeMe" />
        </label>
        <button onClick={this.handleClick}>Read the value</button>
      </div>
    );
  }
}

ReactDOM.render(
  <NameForm />,
  document.getElementById('root')
);

<div id="root"></div>

Rendered by WebCode

Note that in React, you define the default value with the attribute defaultValue and not value.

Usage

Setting a global runtime value

Referencing a value that will persist and stay the same on the next renders (React component or React HTML element) to be able to manipulate them.

Avoiding new initialization

Avoiding new initialization of value or function. React performs the initialization of the ref value once and ignores it on the next renders.

Manipulating the DOM

A Parent component can set a reference on one more child React elements.

Note that once a component unmounts, React:

set ref.current to null if ref is a value
or call the ref function with null if ref is a callback function.

Due to the DOM manipulation, you can:

track mount and unmount of component
Manage focus, text selection, or media playback.
Trigger imperative animations.
Integrating with non-React systems such as third-party DOM libraries or the built-in browser APIs.

Example: In this example:

we set a ref to an input html react element
we change its value via the DOM element that is accessible through the Ref

const Input = function() {

  const input = React.useRef();

  React.useEffect(function(){
    console.log('The DOM element is a '+ input.current.__proto__.toString());
    const newValue = "Use Effect Value";
    console.log('We can change its value to ('+newValue+') via the DOM element');
    input.current.value = newValue;
  },[input])

  return (
      <input type="text" ref={input} value="first Render value"/>
  );
    
}

Rendered by WebCode

Management

Creation

Refs are created using:

React.createRef() (since 16.3)
or the hook for function component

Refs are commonly assigned to an instance property when a component is constructed so they can be referenced throughout the component.

Refs are attached to React elements via the ref attribute.

Something like that:

const ref = React.createRef();
return ( 
   <input type="text" ref={ref} defaultValue="ChangeMe" />
)

Hook

React has a Hook to create Ref. See useRef