Hey React devs! Today, we’re diving into the useMemo hook - a tool that can speed up our apps but can also complicate things if not used wisely.

So, what’s the deal with useMemo? Well, it's a hook that helps us optimize function executions by “remembering” the previous results, and only redoing the work if the inputs change. It’s like having a super-smart assistant that remembers everything, ensuring our app runs faster and smoother. Handy, right?

But here’s a little story. A while back, someone on my team got really into useMemo and started using it everywhere. I mean, everywhere! And while it’s fabulous, it’s not supposed to be a one-size-fits-all solution; otherwise, we’d have a useWithoutMemo hook built into React!

To save you from the useMemo everywhere trap, I've compiled this guide where we explore the best spots and not-so-great spots to use useMemo. Let's jump in!

When to Use useMemo

useMemo shines in these scenarios:

1. Complex Calculations

When your component deals with heavy computations, useMemo ensures these only re-run when necessary, keeping the performance sleek.

function complexCalculation(list) {
  // Complex calculation here
  return result;
}

function MyComponent({ list }) {
  const memoizedResult = useMemo(() => {
    return complexCalculation(list)
  }, [list]);

  return <div>{memoizedResult}</div>;
};

2. Maintaining Referential Equality

useMemo helps in keeping the same reference for objects or arrays when passing them as props, avoiding needless renders.

function MyComponent({ values }) {
  const memoizedValues = useMemo(() => values, [values]);

  return <MemoizedChildComponent values={memoizedValues} />;
};

function ChildComponent({ values }) {
  // ...
});

const MemoizedChildComponent = React.memo(ChildComponent)

3. Filtering or Sorting Large Lists

When working with large datasets, using useMemo for filtering or sorting prevents these operations from rerunning during every render, saving precious computing power.

function MyComponent({ list }) {
  const filteredList = useMemo(() => {
    return list.filter(item => item.active);
  }, [list]);

  return <ListDisplay items={filteredList} />;
};

When Not to Use useMemo

useMemo is not always the hero we deserve. Here’s when it can be counterproductive:

1. Simple Calculations

For basic calculations, bringing in useMemo can complicate things more than helping.

function MyComponent({ value }) {
  // Not recommended for simple calculations
  const multipliedValue = useMemo(() => {
    return value * 2;
  }, [value]);

  return <div>{multipliedValue}</div>;
};

2. Working with Primitive Values

Memoizing primitive values, like numbers or strings, is generally overkill as they don't cause unnecessary renders.

function MyComponent({ value }) {
  // Not recommended for primitive values
  const memoizedValue = useMemo(() => value + 1, [value]);

  return <div>{memoizedValue}</div>;
};

### 3. Frequent Changes in Dependencies

If the inputs change a lot, `useMemo` loses its efficiency because it ends up recalculating often.

function MyComponent({ frequentlyChangingValue }) {
  // Not recommended for frequently changing dependencies
  const memoizedValue = useMemo(() => {
    return expensiveCalculation(frequentlyChangingValue);
  }, [frequentlyChangingValue]);

  return <div>{memoizedValue}</div>;
};

Conclusion

Remember, it’s not about using useMemo everywhere, but about using it wisely. Start with the simple approach first, and then optimize as needed, avoiding premature optimization.

I hope this guide sets you on the path to creating more efficient React apps. Happy coding!