Async once

asyncOnce.js

/**
 * Takes a generator function that can yeild async functions.
 * When the wrapped generator is called again, the previous call will cancel ASAP.
 * Note: it won't cancel in-flight requests made (see: AbortController)
 * @param {GeneratorFunction} generator
 * @see https://dev.to/chromiumdev/cancellable-async-functions-in-javascript-5gp7
 * @author Sam Thorogood
 * @returns {(...args: any[]) => Promise<any>}
 */
export const asyncOnce = generator => {
  let globalNonce
  return async function(...args) {
    const localNonce = new Object()
    globalNonce = localNonce

    const iter = generator(...args)
    let resumeValue
    for (;;) {
      const n = iter.next(resumeValue)
      if (n.done) {
        return n.value // final return value of passed generator
      }

      // whatever the generator yielded, _now_ run await on it
      resumeValue = await n.value
      if (localNonce !== globalNonce) {
        return // a new call was made
      }
      // next loop, we give resumeValue back to the generator
    }
  }
}