node/lib/internal/process/next_tick.js

'use strict';

exports.setup = setupNextTick;

function setupNextTick() {
  const {
    getDefaultTriggerAsyncId,
    newAsyncId,
    initHooksExist,
    destroyHooksExist,
    emitInit,
    emitBefore,
    emitAfter,
    emitDestroy,
    symbols: { async_id_symbol, trigger_async_id_symbol }
  } = require('internal/async_hooks');
  const promises = require('internal/process/promises');
  const { ERR_INVALID_CALLBACK } = require('internal/errors').codes;
  const FixedQueue = require('internal/fixed_queue');
  const { emitPromiseRejectionWarnings } = promises;

  // tickInfo is used so that the C++ code in src/node.cc can
  // have easy access to our nextTick state, and avoid unnecessary
  // calls into JS land.
  // runMicrotasks is used to run V8's micro task queue.
  const [
    tickInfo,
    runMicrotasks
  ] = process._setupNextTick(_tickCallback);

  // *Must* match Environment::TickInfo::Fields in src/env.h.
  const kHasScheduled = 0;
  const kHasPromiseRejections = 1;

  const queue = new FixedQueue();

  process.nextTick = nextTick;
  // Needs to be accessible from beyond this scope.
  process._tickCallback = _tickCallback;

  function _tickCallback() {
    let tock;
    do {
      while (tock = queue.shift()) {
        const asyncId = tock[async_id_symbol];
        emitBefore(asyncId, tock[trigger_async_id_symbol]);
        // emitDestroy() places the async_id_symbol into an asynchronous queue
        // that calls the destroy callback in the future. It's called before
        // calling tock.callback so destroy will be called even if the callback
        // throws an exception that is handled by 'uncaughtException' or a
        // domain.
        // TODO(trevnorris): This is a bit of a hack. It relies on the fact
        // that nextTick() doesn't allow the event loop to proceed, but if
        // any async hooks are enabled during the callback's execution then
        // this tock's after hook will be called, but not its destroy hook.
        if (destroyHooksExist())
          emitDestroy(asyncId);

        const callback = tock.callback;
        if (tock.args === undefined)
          callback();
        else
          Reflect.apply(callback, undefined, tock.args);

        emitAfter(asyncId);
      }
      tickInfo[kHasScheduled] = 0;
      runMicrotasks();
    } while (!queue.isEmpty() || emitPromiseRejectionWarnings());
    tickInfo[kHasPromiseRejections] = 0;
  }

  class TickObject {
    constructor(callback, args, triggerAsyncId) {
      // this must be set to null first to avoid function tracking
      // on the hidden class, revisit in V8 versions after 6.2
      this.callback = null;
      this.callback = callback;
      this.args = args;

      const asyncId = newAsyncId();
      this[async_id_symbol] = asyncId;
      this[trigger_async_id_symbol] = triggerAsyncId;

      if (initHooksExist()) {
        emitInit(asyncId,
                 'TickObject',
                 triggerAsyncId,
                 this);
      }
    }
  }

  // `nextTick()` will not enqueue any callback when the process is about to
  // exit since the callback would not have a chance to be executed.
  function nextTick(callback) {
    if (typeof callback !== 'function')
      throw new ERR_INVALID_CALLBACK();

    if (process._exiting)
      return;

    var args;
    switch (arguments.length) {
      case 1: break;
      case 2: args = [arguments[1]]; break;
      case 3: args = [arguments[1], arguments[2]]; break;
      case 4: args = [arguments[1], arguments[2], arguments[3]]; break;
      default:
        args = new Array(arguments.length - 1);
        for (var i = 1; i < arguments.length; i++)
          args[i - 1] = arguments[i];
    }

    if (queue.isEmpty())
      tickInfo[kHasScheduled] = 1;
    queue.push(new TickObject(callback, args, getDefaultTriggerAsyncId()));
  }
}