A very quick-and-dirty proxy interceptor for introspection of an object's behaviour. Assumes chalk is installed.

intercept.js

import chalk from 'chalk';

function str(val) {
  return val ? val.toString().replace(/\s+/g, ' ') : val;
}

let i = 0;
export function intercept(obj, title) {
  return new Proxy(obj, {
    get(target, name) {
      const value = target[name];
      if(name === 'toString') return value;
      if(typeof value !== 'function') return value;
      return new Proxy(value, {
        get(target, name1) {
          if(name1 === 'toString') {
            return () => str(target);
          }
          return target[name1];
        },
        apply(target, thisArg, args) {
          const k = ++i;
          console.log(chalk.blue(`EXEC [${k}]  => ${title}.${name}(${args.map(str).join(', ')})`));
          const value = target.apply(thisArg, args);
          console.log(chalk.gray(`EXEC [${k}] <=  ${chalk.green(str(value))}`));
          return value;
        }
      });
    },
    set(target, name, value) {
      console.log(chalk.magenta(`SET ${title}.${name} = ${str(value)}`));
      target[name] = value;
      return true;
    }
  });
}

sample.js

import {intercept} from './intercept';

const timer = intercept(new VirtualTimer(), 'timer');
const sink = new Sink();
const testEnv = new TestEnv(timer, sink);
const disposable = new SettableDisposable();
const observer = new Observer(sink.event.bind(sink), sink.end.bind(sink), sink.error.bind(sink), disposable);
const scheduler = intercept(new Scheduler(timer), 'scheduler');
disposable.setDisposable(source.run(observer, scheduler));

// We've intercepted scheduler and timer above. The output below will display everything they do.

sample.output.txt

## Note that the following will be colored, thanks to chalk, making it easier to scan through.

EXEC [1]  => scheduler.asap([object Object])
EXEC [2]  => scheduler.schedule(0, -1, [object Object])
EXEC [3]  => scheduler.now()
EXEC [4]  => timer.now()
EXEC [4] <=  0
EXEC [3] <=  0
EXEC [5]  => scheduler._scheduleNextRun(0)
EXEC [6]  => scheduler._scheduleNextArrival(0, 0)
SET scheduler._nextArrival = 0
EXEC [7]  => timer.setTimer(function () { self._runReadyTasks(self.now()); }, 0)
SET timer._task = function () { self._runReadyTasks(self.now()); }
SET timer._time = 0
EXEC [7] <=  [object Object]
SET scheduler._timer = [object Object]
EXEC [6] <=  undefined
EXEC [5] <=  undefined
EXEC [2] <=  [object Object]
EXEC [1] <=  [object Object]
NEXT
EXEC [8]  => timer.tick(10)
SET timer._targetNow = 10
EXEC [9]  => timer._run()
SET timer._active = true
EXEC [10]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [10] <=  [object Promise]
SET timer._promise = [object Promise]
EXEC [9] <=  [object Promise]
EXEC [8] <=  [object Promise]
SET timer._task = undefined
SET timer._now = 0
SET timer._time = Infinity
EXEC [11]  => timer._task()
EXEC [12]  => scheduler._runReadyTasksBound()
EXEC [13]  => timer.now()
EXEC [13] <=  0
EXEC [14]  => scheduler.asap([object Object])
EXEC [15]  => scheduler.schedule(0, -1, [object Object])
EXEC [16]  => scheduler.now()
EXEC [17]  => timer.now()
EXEC [17] <=  0
EXEC [16] <=  0
EXEC [18]  => scheduler._scheduleNextRun(0)
EXEC [19]  => scheduler._scheduleNextArrival(0, 0)
SET scheduler._nextArrival = 0
EXEC [20]  => timer.setTimer(function () { self._runReadyTasks(self.now()); }, 0)
SET timer._task = function () { self._runReadyTasks(self.now()); }
SET timer._time = 0
EXEC [21]  => timer._run(function () { self._runReadyTasks(self.now()); })
EXEC [21] <=  [object Promise]
EXEC [20] <=  [object Object]
SET scheduler._timer = [object Object]
EXEC [19] <=  undefined
EXEC [18] <=  undefined
EXEC [15] <=  [object Object]
EXEC [14] <=  [object Object]
EXEC [22]  => scheduler.asap([object Object])
EXEC [23]  => scheduler.schedule(0, -1, [object Object])
EXEC [24]  => scheduler.now()
EXEC [25]  => timer.now()
EXEC [25] <=  0
EXEC [24] <=  0
EXEC [26]  => scheduler._scheduleNextRun(0)
EXEC [26] <=  undefined
EXEC [23] <=  [object Object]
EXEC [22] <=  [object Object]
EXEC [27]  => timer.now()
EXEC [27] <=  0
EXEC [12] <=  undefined
EXEC [11] <=  undefined
EXEC [28]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [28] <=  [object Promise]
SET timer._task = undefined
SET timer._now = 0
SET timer._time = Infinity
EXEC [29]  => timer._task()
EXEC [30]  => scheduler._runReadyTasksBound()
EXEC [31]  => timer.now()
EXEC [31] <=  0
EXEC [32]  => scheduler.delay(10, [object Object])
EXEC [33]  => scheduler.schedule(10, -1, [object Object])
EXEC [34]  => scheduler.now()
EXEC [35]  => timer.now()
EXEC [35] <=  0
EXEC [34] <=  0
EXEC [36]  => scheduler._scheduleNextRun(0)
EXEC [37]  => scheduler._scheduleNextArrival(10, 0)
SET scheduler._nextArrival = 10
EXEC [38]  => timer.setTimer(function () { self._runReadyTasks(self.now()); }, 10)
SET timer._task = function () { self._runReadyTasks(self.now()); }
SET timer._time = 10
EXEC [39]  => timer._run(function () { self._runReadyTasks(self.now()); })
EXEC [39] <=  [object Promise]
EXEC [38] <=  [object Object]
SET scheduler._timer = [object Object]
EXEC [37] <=  undefined
EXEC [36] <=  undefined
EXEC [33] <=  [object Object]
EXEC [32] <=  [object Object]
EXEC [40]  => scheduler.delay(10, [object Object])
EXEC [41]  => scheduler.schedule(10, -1, [object Object])
EXEC [42]  => scheduler.now()
EXEC [43]  => timer.now()
EXEC [43] <=  0
EXEC [42] <=  0
EXEC [44]  => scheduler._scheduleNextRun(0)
EXEC [44] <=  undefined
EXEC [41] <=  [object Object]
EXEC [40] <=  [object Object]
EXEC [45]  => scheduler.delay(5, [object Object])
EXEC [46]  => scheduler.schedule(5, -1, [object Object])
EXEC [47]  => scheduler.now()
EXEC [48]  => timer.now()
EXEC [48] <=  0
EXEC [47] <=  0
EXEC [49]  => scheduler._scheduleNextRun(0)
EXEC [50]  => scheduler._unschedule()
EXEC [51]  => timer.clearTimer([object Object])
EXEC [52]  => timer._cancel()
SET timer._timer = null
EXEC [52] <=  undefined
SET timer._time = Infinity
SET timer._task = undefined
EXEC [51] <=  undefined
SET scheduler._timer = null
EXEC [50] <=  undefined
EXEC [53]  => scheduler._scheduleNextArrival(5, 0)
SET scheduler._nextArrival = 5
EXEC [54]  => timer.setTimer(function () { self._runReadyTasks(self.now()); }, 5)
SET timer._task = function () { self._runReadyTasks(self.now()); }
SET timer._time = 5
EXEC [55]  => timer._run(function () { self._runReadyTasks(self.now()); })
EXEC [55] <=  [object Promise]
EXEC [54] <=  [object Object]
SET scheduler._timer = [object Object]
EXEC [53] <=  undefined
EXEC [49] <=  undefined
EXEC [46] <=  [object Object]
EXEC [45] <=  [object Object]
EXEC [56]  => scheduler.delay(5, [object Object])
EXEC [57]  => scheduler.schedule(5, -1, [object Object])
EXEC [58]  => scheduler.now()
EXEC [59]  => timer.now()
EXEC [59] <=  0
EXEC [58] <=  0
EXEC [60]  => scheduler._scheduleNextRun(0)
EXEC [60] <=  undefined
EXEC [57] <=  [object Object]
EXEC [56] <=  [object Object]
EXEC [61]  => timer.now()
EXEC [61] <=  0
EXEC [30] <=  undefined
EXEC [29] <=  undefined
EXEC [62]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [62] <=  [object Promise]
SET timer._task = undefined
SET timer._now = 5
SET timer._time = Infinity
EXEC [63]  => timer._task()
EXEC [64]  => scheduler._runReadyTasksBound()
EXEC [65]  => timer.now()
EXEC [65] <=  5
EVENT! 50
EXEC [66]  => scheduler.cancelAll(function (task) { return task.sink === self.sink; })
EXEC [67]  => scheduler._reschedule()
EXEC [68]  => scheduler.now()
EXEC [69]  => timer.now()
EXEC [69] <=  5
EXEC [68] <=  5
EXEC [70]  => scheduler._scheduleNextRun(5)
EXEC [71]  => scheduler._scheduleNextArrival(10, 5)
SET scheduler._nextArrival = 10
EXEC [72]  => timer.setTimer(function () { self._runReadyTasks(self.now()); }, 5)
SET timer._task = function () { self._runReadyTasks(self.now()); }
SET timer._time = 10
EXEC [73]  => timer._run(function () { self._runReadyTasks(self.now()); })
EXEC [73] <=  [object Promise]
EXEC [72] <=  [object Object]
SET scheduler._timer = [object Object]
EXEC [71] <=  undefined
EXEC [70] <=  undefined
EXEC [67] <=  undefined
EXEC [66] <=  undefined
EXEC [74]  => scheduler.cancel([object Object])
EXEC [74] <=  undefined
EXEC [75]  => timer.now()
EXEC [75] <=  5
EXEC [64] <=  undefined
EXEC [63] <=  undefined
EXEC [76]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [76] <=  [object Promise]
SET timer._task = undefined
SET timer._now = 10
SET timer._time = Infinity
EXEC [77]  => timer._task()
EXEC [78]  => scheduler._runReadyTasksBound()
EXEC [79]  => timer.now()
EXEC [79] <=  10
EVENT! 100
EXEC [80]  => scheduler.cancelAll(function (task) { return task.sink === self.sink; })
EXEC [81]  => scheduler._reschedule()
EXEC [82]  => scheduler._unschedule()
EXEC [83]  => timer.clearTimer()
EXEC [83] <=  undefined
SET scheduler._timer = null
EXEC [82] <=  undefined
EXEC [81] <=  undefined
EXEC [80] <=  undefined
EXEC [84]  => scheduler.cancel([object Object])
EXEC [84] <=  undefined
EXEC [85]  => timer.now()
EXEC [85] <=  10
EXEC [78] <=  undefined
EXEC [77] <=  undefined
EXEC [86]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [86] <=  [object Promise]
END!
SET timer._now = 10
SET timer._time = Infinity
SET timer._promise = null
SET timer._promise = null
SET timer._promise = null
SET timer._promise = null
SET timer._promise = null
NEXT
Bucket { t: 10, events: [ 50, 100 ], error: null, end: { x: undefined } }
NEXT
EXEC [87]  => timer.tick(1)
SET timer._targetNow = 11
EXEC [88]  => timer._run()
SET timer._active = true
EXEC [89]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [89] <=  [object Promise]
SET timer._promise = [object Promise]
EXEC [88] <=  [object Promise]
EXEC [87] <=  [object Promise]
SET timer._task = undefined
SET timer._now = Infinity
SET timer._time = Infinity
EXEC [90]  => timer._step()
SET timer._timer = [object Object]
SET timer._promise = [object Promise]
EXEC [90] <=  [object Promise]
SET timer._now = 11
SET timer._time = Infinity
SET timer._promise = null
SET timer._promise = null
NEXT