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useEffect

What is useEffect

Performs side effects on your components.

Side effect is a code that changes things outside the component's return definition.

  • e.g. setting cookies or modifying HTML.
useEffect(callback, dependency)

Also used to manage lifecycle of a component.

  • When you define a function in a component, defined function is executed during the rendering phase of the component (not before or after the render).

useEffect combines the functionality of componentDidMount, componentDidUpdate, componentWillUnmount of Class React components.

dependency

useEffect is used to run some sort of logic after the render.

Used to call some logic specifically after a dependency is updated.

You can control when these functions will be called and can get a guarantee that some functions will run after the rendering is complete (meaning, the DOM will be accessible).

useEffect(() => {
  // runs on every render
});

useEffect(() => {
  // runs only on the first render
}, []);

useEffect(() => {
  // runs on the first render
  // and any time any dependency value changes
}, [prop, state]);

useEffect(() => {
  return () => {
    cleanup(); // cleanup function
  };
}, []);

Without dependencies, the useEffect hook runs on mount and on every re-render.

With an array of dependencies, the useEffect hook runs on mount and runs on subsequent renders when one or all of its dependencies change.

Dependency Array

With an empty array of dependencies, the useEffect hook is called once on mount.

Without dependencies, the useEffect hook runs on mount and on every re-render

With an array of dependencies, the useEffect hook runs on mount and runs on subsequent renders when any of its dependencies change.

With an empty array of dependencies, the useEffect Hook is called once on mount.

Infinite Re-render

If the variable referenced in the dependency array is NOT primitive, you will likely run into an infinite render loop:

  • The useEffect internally compares the dependencies by reference.
const [count, setCount] = useState(0);

useEffect(() => {
  setCalculation(() => count * 2);
}, [count]); // count is a primitive number variable

const [someObj, setSomeObj] = useState({});

useEffect(() => {
  // new object passed every time with new address
  setSomeObj({field1: "this", field2: "that"});
}, [someObj])

Solution to infinite rendering problem:

const [someObj, setSomeObj] = useState({});
const { field1, field2 } = someObj;

useEffect(() => {
  setSomeObj({field1: "this", field2: "that"});
}, [field1, field2]); // manually passing each field which is primitive
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