Keep object chainable using async methods

Question

Let's say I have a class Test with around 10-20 methods, all of which are chainable.

In another method, I have some asynchronous work to do.

let test = new Test();
console.log(test.something()); // Test
console.log(test.asynch()); // undefined since the async code isn't done yet
console.log(test.asynch().something()); // ERROR > My goal is to make this 

Since every other method is chainable, I feel like it would be weird for the user if this sole method isn't.

Is there a way for me to maintain the chainable theme of my Class?

---

I have already thought of passing the next method in a callback function inside this method's parameter, but it's not really chaining.

test.asynch(() => something())

Same thing with Promises, it's not really chaining.

test.asynch().then(() => something())

The result I want is

test.asynch().something()

---

Here is a snippet that demonstrates my issue :

class Test {
  /**
   * Executes some async code
   * @returns {Test} The current {@link Test}
   */
  asynch() {
    if (true) { //Condition isn't important
      setTimeout(() => { //Some async stuff
        return this;
      }, 500);
    } else {
      // ...
      return this;
    }
  }

  /**
   * Executes some code
   * @returns {Test} The current {@link Test}
   */
  something() {
    // ...
    return this
  }
}

let test = new Test();
console.log(test.something()); // Test
console.log(test.asynch()); // undefined
console.log(test.asynch().something()); // ERROR > My goal is to make this work.

Answer 1

I doupt that it is a really good idea to do something like that. But using a Proxy would allow to create such a beahviour if the original Object meets certain conditions. And I would highly recommend to not do it that way.

Be aware that this code is a proof of concept to show that it is somehow possible, but doesn't care about edge cases and most likely will break certains functionalities.

One Proxy is used to wrap the original class Test so that is is possible to patch each of is instance to make them chainable.

The second one will patch each function call and creates a queue, for these functions calls so that they are called in order.

    class Test {
      /**
       * Executes some async code
       * @returns {Test} The current {@link Test}
       */
      asynch() {
        console.log('asynch')
        return new Promise((resolve, reject) => setTimeout(resolve, 1000))
      }

      /**
       * Executes some code
       * @returns {Test} The current {@link Test}
       */
      something() {
        console.log('something')

        return this
      }
    }


    var TestChainable = new Proxy(Test, {
      construct(target, args) {
        return new Proxy(new target(...args), {

          // a promise used for chaining
          pendingPromise: Promise.resolve(),

          get(target, key, receiver) {
            //  intercept each get on the object
            if (key === 'then' || key === 'catch') {
              // if then/catch is requested, return the chaining promise
              return (...args2) => {
                return this.pendingPromise[key](...args2)
              }
            } else if (target[key] instanceof Function) {
              // otherwise chain with the "chainingPromise" 
              // and call the original function as soon
              // as the previous call finished 
              return (...args2) => {
                this.pendingPromise = this.pendingPromise.then(() => {
                  target[key](...args2)
                })

                console.log('calling ', key)

                // return the proxy so that chaining can continue
                return receiver
              }
            } else {
              // if it is not a function then just return it
              return target[key]
            }
          }
        })
      }
    });

    var t = new TestChainable
    t.asynch()
      .something()
      .asynch()
      .asynch()
      .then(() => {
        console.log('all calles are finished')
      })

Answer 2

I don't think that is it possible to use such syntax for now. It would require to access the promise in the in the function to return it.

Different ways to chain functions:

Promise with then

bob.bar()
    .then(() => bob.baz())
    .then(() => bob.anotherBaz())
    .then(() => bob.somethingElse());

And you could also use compositions, to obtain another style of functional, reusable syntax to chain async and sync functions

const applyAsync = (acc,val) => acc.then(val);
const composeAsync = (...funcs) => x => funcs.reduce(applyAsync, Promise.resolve(x));
const transformData = composeAsync(func1, asyncFunc1, asyncFunc2, func2);
transformData(data);

Or using async / await

for (const f of [func1, func2]) {
  await f();
}

Answer 3

As discussed in comments to OP, this can be accomplished by using Proxy.

I recognize that t.niese provided a similar answer a few hours ago. My approach differs somewhat, but it's still substantively trapping method calls, returning the receiver and internally stacking up thennables.

class ProxyBase {

    constructor () {

        // Initialize a base thennable.
        this.promiseChain = Promise.resolve();

    }

    /**
     * Creates a new instance and returns an object proxying it.
     * 
     * @return {Proxy<ProxyBase>}
     */
    static create () {

        return new Proxy(new this(), {

            // Trap all property access.
            get: (target, propertyName, receiver) => {

                const value = target[propertyName];

                // If the requested property is a method and not a reserved method...
                if (typeof value === 'function' && !['then'].includes(propertyName)) {

                    // Return a new function wrapping the method call.
                    return function (...args) {

                        target.promiseChain = target.promiseChain.then(() => value.apply(target, args));

                        // Return the proxy for chaining.
                        return receiver;

                    }

                } else if (propertyName === 'then') {
                    return (...args) => target.promiseChain.then(...args);
                }

                // If the requested property is not a method, simply attempt to return its value.
                return value;

            }

        });

    }

}

// Sample implementation class. Nonsense lies ahead.
class Test extends ProxyBase {

    constructor () {
        super();
        this.chainValue = 0;
    }

    foo () {
        return new Promise(resolve => {
            setTimeout(() => {
                this.chainValue += 3;
                resolve();
            }, 500);
        });
    }

    bar () {
        this.chainValue += 5;
        return true;
    }

    baz () {
        return new Promise(resolve => {
            setTimeout(() => {
                this.chainValue += 7;
                resolve();
            }, 100);
        });
    }

}

const test = Test.create();

test.foo().bar().baz().then(() => console.log(test.chainValue)); // 15