Make await return thenable, or infinite recursion in awaiting awaitable

Question

Trying to come up with API, mixing in promise functionality like:

class Awaitable {
  constructor () {
    this.promise = Promise.resolve()
  }
  then (fn) {
    // awaited result must be _this_ instance
    return this.promise.then(() => fn(this))
  }
}

let smth = await (new Awaitable())
console.log(smth)

This code creates recursion. The main point is to have smth to be the newly created thenable instance.

Stubbing then with null makes awaited result incomplete.

I wonder if that's possible at all, seems like there's some conceptual hurdle, I can't wrap my head around.

Answer 1

The correct solution

Symbol.thenable proposal.

import { parse as parseStack } from 'stacktrace-parser' 

class Awaitable {
  constructor () {
    this.promise = Promise.resolve()
  },

  [Symbol.thenable]: false,

  then(fn) {
    this.promise.then(() => fn(this))
    return this
  }
}

let smth = await (new Awaitable())
console.log(smth.then) // function

Fragile non-standard solution

Detecting if thenable instance is called by await is possible by parsing callstack. Here's solution based on stacktrace-parser package:

import { parse as parseStack } from 'stacktrace-parser' 

class Awaitable {
  constructor () {
    this.promise = Promise.resolve()
  }
}

Object.defineProperty(Awaitable.prototype, 'then', {
  get() {
    let stack = parseStack((new Error).stack)
    
    // naive criteria: if stacktrace is leq 3, that's async recursion, bail out
    // works in webkit, FF/nodejs needs better heuristic
    if (stack.length <= 3) return null
    
    return (fn) => {
      this.promise.then(() => {
        fn(this)
      })
      return this
    }
  }
})

let smth = await (new Awaitable())
console.log(smth.then) // function

The heuristic must be enhanced for FF/nodejs, to solidify - that'd require sort of static analysis wizardy.