createPromiseSingletonContext.ts

export const PROMISE_SINGLETON_REJECTION_REASON = 'rejected by successor';

/**
 * Creates a context where by only the last called promise is resolved
 * Previous ones would be rejected
 *
 * @example
 *
 * const context = createPromiseSingletonContext<string>();
 * const someApiCall = (result: string) => new Promise<string>(resolve => {
 *    setTimeout(() => resolve(result), 1000)
 *  });
 *
 * let result: string;
 * let rejectedErr: string;
 *
 * const call = async (attempt: string) => {
 *    try {
 *
 *      result = await context(() => someApiCall('one'));
 *
 *    } catch (err) {
 *      // You might abort a fetch here if needed
 *      rejectedErr = err;
 *    }
 * }
 *
 * call('one');
 * await call('two');
 *
 * > result
 * < "two"
 *
 * > rejectedErr
 * < "rejected by successor"
 */
export function createPromiseSingletonContext<TResult>() {
  let previous: (reason: string) => void;
  return (action: () => Promise<TResult>) => {
    if (previous) {
      previous(PROMISE_SINGLETON_REJECTION_REASON);
      previous = null;
    }
    return new Promise<TResult>(async (resolve, reject) => {
      previous = reject;
      resolve(await action());
    });
  };
}