LazyPromise

The LazyPromise is PromiseLike object that behaves similarly to regular Promise but executes only when awaited or when manually defered.

Basic usage

Creating and executing

import { LazyPromise } from "@daiso-tech/core/async";

const promise = new LazyPromise(async () => {
    // This will log when awaited
    console.log("2.");
    return await fetch("URL");
});

console.log("1.");
await promise;
console.log("3.");

You can execute a LazyPromise without awaiting it:

import { LazyPromise } from "@daiso-tech/core/async";
import { TimeSpan } from "@daiso-tech/core/utilities";

const promise = new LazyPromise(async () => {
    LazyPromise.delay(TimeSpan.fromSeconds(1));
    console.log("3.");
    return await fetch("URL");
});

console.log("1.");
promise.defer();
console.log("2.");

Adding middlewares

You can add middlewares to a LazyPromise. Refer to the Hooks documentation for further information.

Through the constructor:

import { LazyPromise } from "@daiso-tech/core/async";

const promise = new LazyPromise(async () => {
    return await fetch("URL");
}, [
    (args, next) => {
        console.log("Middleware 1");
        return next(...args);
    },
]);

await promise;

Through the pipe method:

import { LazyPromise } from "@daiso-tech/core/async";

const promise = new LazyPromise(async () => {
    return await fetch("URL");
}).pipe((args, next) => {
    console.log("Middleware 1");
    return next(...args);
});

await promise;

Through the pipeWhen method:

import { LazyPromise } from "@daiso-tech/core/async";

const promise = new LazyPromise(async () => {
    return await fetch("URL");
}).pipeWhen(true, (args, next) => {
    console.log("Middleware 1");
    return next(...args);
});

await promise;

Static methods

LazyPromise provides static helper methods (all, allSettled, race, and any) that mirror their Promise counterparts, but with lazy execution behavior. Like individual LazyPromise instances, these methods only evaluate when awaited or when their defer method is explicitly called.

The are additional static method, delay and fromCallback.

• The delay method creates a LazyPromise that resolves after a given time:

  import { LazyPromise } from "@daiso-tech/core/async";
  import { TimeSpan } from "@daiso-tech/core/utilities";
  
  // Will wait 1 second
  await LazyPromise.delay(TimeSpan.fromSeconds(1));
  console.log("DONE");

  You can provide an AbortSignal to aborat the delay:

  import { LazyPromise } from "@daiso-tech/core/async";
  import { TimeSpan } from "@daiso-tech/core/utilities";
  
  const abortController = new AbortController();
  abortController.abort("No need to wait");
  
  // Will not wait for the delay
  await LazyPromise.delay(TimeSpan.fromSeconds(1, abortController.signal));
  console.log("DONE");

• The fromCallback is convience method used for wrapping Node js callback functions with a LazyPromise.

  import { LazyPromise } from "@daiso-tech/core/async";
  import { readFile } from "node:fs";
  
  const lazyPromise = LazyPromise.fromCallback<Buffer | string>((resolve, reject) => {
      readFile("FILE_PATH", (err, data) => {
          if (err !== null) {
              reject(err);
              return;
          }
          resolve(data);
      });
  });
  const file = await lazyPromise;
  console.log(file);