React Hooks: useRef

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1. Definition

useRef is a built-in React hook that accepts one argument as the initial value and returns a reference (aka ref).

A reference is an object having a special property current

There are two main uses of useRef;

Accessing the DOM nodes or React elements
Keeping a mutable variable.

const refContainer = useRef(initialValue);

2. Accessing the DOM Elements

This is performed in 3 steps:

Define the reference to access the element
Assign the reference to ref attribute of the element
After mounting, elementRef.current points to the DOM element.

This is equivalent of

import React, { useRef } from "react";

const CustomTextInput = () => {
  const textInput = useRef();

  focusTextInput = () => textInput.current.focus();

  return (
    <>
      <input type="text" ref={textInput} />
      <button onClick={focusTextInput}>Focus the text input</button>
    </>
  );
}

3. Keeping a Mutable Variable

We have two ways of keeping data between re-renders:

In the component state: Every time the state changes, the component will be re-rendered.
In an instance variable: The variable will persist for the full lifetime of the component. Changes in an instance variable won’t generate a re-render.

In functional components this would be;

In a state variable: useState or useReducer. Updates in state variables will cause a re-render of the component.
In a ref: Equivalent to instance variables in class components. Mutating the .current property won’t cause a re-render.

4. Updating Ref

Updating a ref variable is a side effect therefore needs to be done inside useEffect or useLayoutEffect

In React all side effect should be inside Layout Phase.

import React, { useRef } from "react";

const RenderCounter = () => {
  const counter = useRef(0);

  useEffect(() => {
    // Every time the component has been re-rendered,
    // the counter is incremented
    counter.current = counter.current + 1;
  }); 

  return (
    <h1>{`The component has been re-rendered ${counter} times`}</h1>
  );
};

5. Summary

The useRef Hook lets us create mutable variables inside functional components.

There are three main key points that you should keep in mind when using the useRef Hook;

A ref created with useRef will be created only when the component has been mounted and preserved for the full lifecycle.
Refs can be used for accessing DOM nodes or React elements, and for storing mutable variables (like with instance variables in class components).
Updating a ref is a side effect so it should be done only inside a useEffect (or useLayoutEffect) or inside an event handler.