This article was originally posted on MoonHighway.com.

Rendering is the heartbeat of a React application. When something changes (props, state), the component tree re-renders, reflecting that change in the user interface. But what happens when we need to do something after a render? As you might imagine, there's a Hook for that.

Consider a simple component, the Checkbox. We're using useState to set a checked value and a function to change the value of checked called setChecked. A user can check and uncheck the box, but how might we alert the user that the box has been checked? We'll try this with an alert as it is a great way to block the thread:

function Checkbox() {
  const [checked, setChecked] = useState(false);

  alert(`checked: ${checked.toString()}`);

  return (
    <>
      <input
        type="checkbox"
        value={checked}
        onChange={() => setChecked(checked => !checked)}
      />
      {checked ? "checked" : "not checked"}
    </>
  );
}

We've added the alert before the render to block the render. The component will not render until the user clicks the OK button on the alert box. Because the alert is blocking, we don't see the next state of the checkbox rendered until clicking OK.

That isn't the goal, so maybe we should place the alert after the return?

function Checkbox() {
  const [checked, setChecked] = useState(false);

  return (
    <>
      <input
        type="checkbox"
        value={checked}
        onChange={() => setChecked(checked => !checked)}
      />
      {checked ? "checked" : "not checked"}
    </>
  );

  alert(`checked: ${checked.toString()}`);
}

Scratch that. We can't call alert after the render because the code will never be reached. To ensure that we see the alert as expected, we can use useEffect. Placing the alert inside of the useEffect function means that the function will be called after the render, as a side effect:

function Checkbox() {
  const [checked, setChecked] = useState(false);

  useEffect(() => {
    alert(`checked: ${checked.toString()}`);
  });

  return (
    <>
      <input
        type="checkbox"
        value={checked}
        onChange={() => setChecked(checked => !checked)}
      />
      {checked ? "checked" : "not checked"}
    </>
  );
}

We use useEffect when a render needs to cause side effects. Think of a side effect as something that a function does that isn't part of the return. The function is the Checkbox. The Checkbox function returns a fragment. But we might want the component to do more than that. Those things we want the component to do other than return UI are called effects.

An alert, a console.log, or an interaction with a browser or native API is not part of the render. It's not part of the return. In a React app though, the render affects the results of one of these events. We can use useEffect to wait for the render, and then provide the values to an alert or a console.log:

useEffect(() => {
  console.log(checked ? "Yes, checked" : "No, not checked");
});

Similarly, we could check in with the value of checked on render and then set that to a value in localStorage:

useEffect(() => {
  localStorage.setItem("checkbox-value", checked);
});

We also might use useEffect to focus on a specific text input that has been added to the DOM. React will render the output, and then call useEffect to focus the element:

useEffect(() => {
  txtInputRef.current.focus();
});

On render, the txtInputRef will have a value. We can access that value in the effect to apply the focus. Every time we render, useEffect has access to the latest values from that render: props, state, refs, etc.

Cool, but... why? Think about a render. We render a checkbox where the checked value is false. On that render, React will look at the value of checked and call useEffect:

useEffect(() => {
  console.log(checked ? "Yes, checked" : "No, not checked");
});

React is calling this function post-render:

useEffect(() => console.log("No, not checked"));

Then we update the checked value to true. This causes another render. At this point, the render will lead to useEffect being called again but at this point the function is different:

useEffect(() => console.log("Yes, checked"));

Every time the component renders, we can see the value of checked in useEffect because useEffect is a unique function every time. Think of useEffect as being a function that happens after a render. When a render fires, we can take a look at that render's values and use them in the effect. Then once we render again, the whole thing starts over. New values, then new renders, then new effects.

useEffect is a powerful tool to understand when building a React application. In the next part of this series, we'll take a closer look at the dependency array that allows us to define more specific rules around why and when rendering should happen.