React Hooks by Example - Part 2

Learn (more) Hooks = Make (more) Money!🤑

We're back to make more Dinero!💵💵💵
Let's explore some of the less commonly used, but nonetheless super-important hooks in react!

The code for all the examples can be found at this Code Sandbox.
This is Part 2 of a three-part series, and it covers useReducer, useRef, useMemo, useCallback, useImperativeHandle and useLayoutEffect!

---

useReducer

• The useReducer can be used as an alternative to useState.
• useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one."

• Example:
  

  // /src/UseReducerExample.js
  
  import { useReducer } from "react";
  import { Link } from "react-router-dom";
  
  const bankBalance = { counter: 0 };
  
  function reducer(state, action) {
    switch (action.type) {
      case "make 1 $":
        return { counter: state.counter + 1 };
      case "spend 1 $":
        return { counter: state.counter - 1 };
      default:
        throw new Error();
    }
  }
  
  export default function UseReducerExample() {
    const [state, dispatch] = useReducer(reducer, bankBalance);
  
    return (
      <>
        <h1>
          <Link to={"/"}>
            Dinero!
          </Link>
        </h1>
        <h2>
          {state.counter >= 0 ? "You have" : "You lost"} {Math.abs(state.counter)}{" "}
          💵
        </h2>
  
        <button onClick={() => dispatch({ type: "spend 1 $" })}>Spend 1 $</button>
        <button onClick={() => dispatch({ type: "make 1 $" })}>Make 1 $</button>
      </>
    );
  }
  

• In the above example:

  • The useReducer hook accepts a reducer function and an data which you want to save to state, here, { counter: 0 }.
  • The useReducer hook returns a state object containing this data, and a dispatch function.
  • When dispatch is called with an action here, { type: "spend 1 $" },
  • It passes the state and this action to the reducer function.
  • Depending on the action type, it will return some value and this value will be saved to the state here, state.counter
  • The updated value of the state can then be accessed inside the component.

• See it in action here and the code at this sandbox!

---

useRef

• The useRef hook can be used to reference a given value, and it will be stored in its .current property between component re-renders, and can be changed.
• A common use-case is to access DOM elements directly.
• It is important to note that mutating .current will not cause a re-render.

• Example:
  

  // /src/UseRefExample.js
  
  import { useRef } from "react";
  import { Link } from "react-router-dom";
  
  export default function UseRefExample() {
    let counter = 0;
    const refAmt = useRef(null);
  
    const updateAmt = () => {
      console.log(refAmt.current);
  
      counter += 1;
      console.log(`You actually have ${counter} 💵`);
    };
  
    return (
      <>
        <h1>
          <Link to={"/"}>
            Dinero!
          </Link>
        </h1>
        <h2 ref={refAmt}>You have {counter} 💵</h2>
        <button onClick={updateAmt}>Get 1 $</button>
      </>
    );
  }
  

• In the above example:

  • We create a ref, refAmt.
  • in the <h2> element we specify ref={refAmt}, so now refAmt.current points to the <h2> element.
  • On clicking the button, we can open the console and see that,
  • The value of counter is increasing.
  • But refAmt.current (which points to the <h2> element) is not being updated with the new value of counter.

• See it in action here and the code at this sandbox!

---

useMemo

• the useMemo hook takes a function (that ideally returns something) and an array of dependencies.
• It will only re-compute the return value if one or more of the dependencies has changed.
• This helps to optimizae expensive computations on each render, that may not necessarily need to be re-computed.
• If no array is provided, a new value will be computed on every render.
• Every value referenced inside the function should also appear in the dependencies array.
• Side effects should be in useEffect, not useMemo.

• Example:
  

  // /src/UseMemoExample.js
  
  import { useState, useMemo } from "react";
  import { Link } from "react-router-dom";
  
  function calculationRes(constant) {
    const result = constant ** 2;
    console.log(`The calculated result is: ${result}`);
    return result;
  }
  
  export default function UseMemoExample() {
    const [counter, setCounter] = useState(0);
    const [constant] = useState(1000000);
    const calculation = useMemo(() => calculationRes(constant), [constant]);
  
    return (
      <>
        <h1>
          <Link to={"/"}>
            Dinero!
          </Link>
        </h1>
        <h2>You have {counter} 💵</h2>
        <button onClick={() => setCounter(counter + 1)}>Get 1 $</button>
        {calculation}
      </>
    );
  }
  

• In the above example:

  • The function calculationRes runs once, the result is memoized and result is logged in console.
  • On clicking on the button, the state counter changes and hence the component re-renders.
  • But on subsequent renders, result is not logged to console.
  • This is because the value of result has not changed since the value of the variable constant has not changed, which was in the dependency array for calculationRes.

• See it in action here and the code at this sandbox!

---

useCallback

• The useCallback hook is very similar to the useMemo hook, the only difference being that useMemo returns a memoized value, and useCallback returns a memoized function.
• A common use-case is when the function needs to be passed down nested child components.

• The example above can be referred to, as an example of useCallback by just replacing:
  
  const calculation = useMemo(() => calculationRes(constant), [constant]);
  with
  

  const calculation = useCallback(() => calculationRes(constant), [constant]);
  

• See it in action here and the code at this sandbox!

---

forwardRef and useImperativeHandle

• As we have seen above, the useRef hook can be used to access DOM elements.
• We can wrap the component exposing this ref, inside forwardRef and then the parent component would have access to this ref.

• Example of forwarding refs:
  

  import { useRef, forwardRef } from "react";
  
  function ChildComponent(props, ref) {
    return (
      <>
        <h1 ref={ref} {...props}>{"💵💵💵💵💵💵💵💵💵💵"}</h1>
      </>
    );
  }
  
  ChildComponent = forwardRef(ChildComponent);
  
  export default function UseImperativeHandleExample() {
    const moneyRef = useRef(null);
  
    return (
      <>
        <ChildComponent ref={moneyRef} />
      </>
    );
  }
  

• In the above example:

  • The parent component UseImperativeHandleExample uses the useRef hook to create a ref object.
  • This ref is pointed to ChildComponent.
  • ChildComponent is wrapped inside forwardRef and hence we can assign this ref object to a DOM element inside it, here, the <h1> element.
  • Now, the parent component UseImperativeHandleExample will have access to the ref to the <h1> element inside ChildComponent.

• Now, if we need to modify the behavious of this ref, based on say, some user action, this is where the useImperativeHandle hook comes in handy!

• The above example can be modified like so:
  

  // /src/UseImperativeHandleExample.js
  
  import { useRef, forwardRef, useImperativeHandle } from "react";
  import { Link } from "react-router-dom";
  
  function ChildComponent(props, ref) {
    const h1Ref = useRef(null);
  
    useImperativeHandle(ref, () => ({
      seeReferencedEl: h1Ref.current
    }));
  
    return (
      <>
        <h1>
          <Link to={"/"}>
            Dinero!
          </Link>
        </h1>
        <h1 ref={h1Ref} {...props}>
          {"💵💵💵💵💵💵💵💵💵💵"}
        </h1>
      </>
    );
  }
  
  ChildComponent = forwardRef(ChildComponent);
  
  export default function UseImperativeHandleExample() {
    const moneyRef = useRef(null);
  
    const seeReferencedElement = () => {
      console.log(moneyRef.current?.seeReferencedEl);
    };
  
    return (
      <>
        <ChildComponent ref={moneyRef} />
        <button onClick={seeReferencedElement}>See ref in console</button>
      </>
    );
  }
  

• In the above example:

  • Inside the UseImperativeHandleExample component, moneyRef.current points to ChildComponent as we saw in the forwardRef example previously.
  • But in this example, we assign a new ref object h1Ref to the <h1> element inside ChildComponent.
  • Using the useImperativeHandle hook, we assign this new h1Ref to the seeReferencedEl property of ChildComponent's ref.
  • So now, inside the UseImperativeHandleExample component, moneyRef.current.seeReferencedEl points to the <h1> element inside ChildComponent.

• See it in action here and the code at this sandbox!

---

useLayoutEffect

• The useLayoutEffect is similar to useEffect, except one key difference.
• It runs (synchronously) after the elements have been loaded into the DOM but before they are painted to the UI.
• "Use this [hook] to read layout from the DOM and synchronously re-render".
• A common use-case is to measure layout and change the position of elements based on it.

• Example:
  

  // /src/UseLayoutEffectExample.js
  import { useRef, useEffect, useLayoutEffect } from "react";
  import { Link } from "react-router-dom";
  
  export default function UseLayoutEffectExample() {
    const refAmt = useRef(null);
  
    useEffect(() => {
      console.log("useEffect called!");
    }, []);
  
    useLayoutEffect(() => {
      console.log("useLayoutEffect called");
    }, []);
  
    return (
      <>
        <h1>
          <Link to={"/"} style={styles.linkStyle}>
            Dinero!
          </Link>
        </h1>
        <h2 ref={refAmt}>You have 1000000 💵</h2>
      </>
    );
  }
  

• In the above example:

  • We see that "useLayoutEffect called" is logged first, since the useLayoutEffect runs synchronously
  • Then the DOM is visually updated.
  • Then "useEffect called!" is logged, since useEffect runs asynchronously.

• See it in action here and the code at this sandbox!

Hope you had great fun learning hooks, and heading to the Buggati store soon for some Sunday shopping!
Stay tuned for Part 3 where we look at more hooks🤘
Cheers!