useHooks(🐠)

One question that comes up a lot is "When using hooks how do I get the previous value of props or state?". With React class components you have the componentDidUpdate method which receives previous props and state as arguments or you can update an instance variable (this.previous = value) and reference it later to get the previous value. So how can we do this inside a functional component that doesn't have lifecycle methods or an instance to store values on? Hooks to the rescue! We can create a custom hook that uses the useRef hook internally for storing the previous value. See the recipe below with inline comments. You can also find this example in the official React Hooks FAQ.

import { useState, useEffect, useRef } from "react";

// Usage
function App() {
  // State value and setter for our example
  const [count, setCount] = useState(0);

  // Get the previous value (was passed into hook on last render)
  const prevCount = usePrevious(count);

  // Display both current and previous count value
  return (
    <div>
      <h1>
        Now: {count}, before: {prevCount}
      </h1>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

// Hook
function usePrevious(value) {
  // The ref object is a generic container whose current property is mutable ...
  // ... and can hold any value, similar to an instance property on a class
  const ref = useRef();

  // Store current value in ref
  useEffect(() => {
    ref.current = value;
  }, [value]); // Only re-run if value changes

  // Return previous value (happens before update in useEffect above)
  return ref.current;
}

import { useState, useEffect, useRef } from "react";

// Usage
function App() {
  // State value and setter for our example
  const [count, setCount] = useState<number>(0);

  // Get the previous value (was passed into hook on last render)
  const prevCount: number = usePrevious<number>(count);

  // Display both current and previous count value
  return (
    <div>
      <h1>
        Now: {count}, before: {prevCount}
      </h1>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

// Hook
function usePrevious<T>(value: T): T {
  // The ref object is a generic container whose current property is mutable ...
  // ... and can hold any value, similar to an instance property on a class
  const ref: any = useRef<T>();

  // Store current value in ref
  useEffect(() => {
    ref.current = value;
  }, [value]); // Only re-run if value changes

  // Return previous value (happens before update in useEffect above)
  return ref.current;
}