React Code Reviews Comments #1: Computed State

Some of the code reviews comments I usually write for junior React developers.

First let's take a look at this component:

const SimpleComponent = () => {
  const [price, setPrice] = React.useState(10);
  const [tax, setTax] = React.useState(0.15);
  const [total, setTotal] = React.useState(price * tax);

  React.useEffect(() => {
    setTotal(price + price * tax);
  }, [price, tax]);

  return (
    <div>
      <input type="number" value={price} onChange={({ target: { value } }) => setPrice(+value)} />
      <span>Total: {total}</span>
    </div>
  );
};

First Code Smell: a state setter setTax is not used, which means tax can be a variable defined out of the component scope, or maybe it's even better to keep it as an optional prop with a default value.

const SimpleComponent = ({ tax = 0.15 }) => {
  const [price, setPrice] = React.useState(10);
  const [total, setTotal] = React.useState(price * tax);

  React.useEffect(() => {
    setTotal(price + price * tax);
  }, [price, tax]);

  return (
    <div>
      <input type="number" value={price} onChange={({ target: { value } }) => setPrice(+value)} />
      <span>Total: {total}</span>
    </div>
  );
};

There may be a special case where you want to keep the initial value of a prop, so you won't need to use the setter, which is fine.

Second Code Smell: A state setter is used only after another state changes.

React.useEffect(() => {
  setTotal(price + price * tax);
}, [price]);

In this example, total changes only after the price state changes, which could be replace with a simple variable

const SimpleComponent = ({ tax = 0.15 }) => {
  const [price, setPrice] = React.useState(10);

  const total = price + price * tax;

  return (
    <div>
      <input type="number" value={price} onChange={({ target: { value } }) => setPrice(+value)} />
      <span>Total: {total}</span>
    </div>
  );
};

By this, we went down from 3 states to 1 state, which makes our component much easier to understand and read.

These may be simple notes, but they can slip in huge components like a data table. Where you should keep which column we are sorting by not the sorted rows, same for filters/pagination.

// Don't
const [sortedRows, setSortedRows] = React.useState(rows);
const handleSortByName = () => {
  setSortedRows(sortRowsBy(rows, "name"));
}

// Do
const [sortBy, setSortBy] = React.useState(null);
const sortedRows = sortRowsBy(rows, sortBy);
const handleSortByName = () => {
  setSortBy("name");
}

sortedRows is what I call a "Computed State" as it changes based on another state.

If it makes performance issues you can always useMemo.

const sortedRows = React.useMemo(() => sortRowsBy(rows, sortBy), [rows, sortBy])

Hey, this is my first blog ever. And it's the first of a series. Please let me know if you have any notes/questions and if you want to see more of this series.