let only last API call get through

When you make an async call on user input then the order the async results resolve may not be the order the user gave the inputs (so not a debounce issue). When user types s then you make an async call with s, then the user types e and you make an async call with se. There are now 2 async calls unresolved, one with s and one with se.

Say the s call takes a second and the se call takes 10 milliseconds then se resolves first and UI is set to result of se but after that s resolves and the UI is set to result of s. You now have an inconsistent UI.

One way to solve this is with debounce and hope you'll never get async calls that last longer than the debounce time but there is no guarantee. Another way is cancel older requests but that is too much of a pain to implement and not supported by all browsers. The way I show below is just to reject the async promise when a newer request was made. So when user types s and e requests s and se are made but when s resolves after se it will be rejected because it was replaced with a newer request.

const REPLACED = 'REPLACED';
const last = (fn) => {
  const current = { value: {} };
  return (...args) => {
    const now = {};
    current.value = now;
    return Promise.resolve(args)
      .then((args) => fn(...args))
      .then((resolve) =>
        current.value === now
          ? resolve
          : Promise.reject(REPLACED)
      );
  };
};
const later = (value, time) =>
  new Promise((resolve) =>
    setTimeout(() => resolve(value), time)
  );
const apiCall = (value) =>
  //slower when value length is 1
  value.length === 1
    ? later(value, 1000) //takes a second
    : later(value, 100); //takes 100ms
const working = last(apiCall);
const Api = ({ api, title }) => {
  const [value, setValue] = React.useState('');
  const [apiResult, setApiResult] = React.useState('');
  React.useEffect(() => {
    api(value).then((resolve) => {
      console.log(title, 'resolved with', resolve);
      setApiResult(resolve);
    });
  }, [api, title, value]);
  return (
    <div>
      <h1>{title}</h1>
      <h3>api result: {apiResult}</h3>
      <input
        type="text"
        value={value}
        onChange={(e) => setValue(e.target.value)}
      />
    </div>
  );
};
const App = () => (
  <div>
    <Api
      api={apiCall}
      title="Broken (type 2 characters fast)"
    />
    <Api api={working} title="Working" />
  </div>
);
ReactDOM.render(<App />, document.getElementById('root'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="root"></div>

let only last API call get through

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