无法使用Node-Cron在module.exports对象中执行回调？

Question

So I am using a module called node-cron to execute a chain of functions every night.

My function is located in another file. I have added required that file in my app.js where the node-cron Object is located. That Object looks like this

var job = new CronJob('00 51 17 * * 0-6', savantJS.getTracksFromSavants, null, true, 'America/Los_Angeles');

This will execute the getTracksFromSavants function in my savantJS file at 5:51pm LA time just fine. However the problem occurs when I try to have callback functions in my savantJS file.

For example:

module.exports = {
  getTracksFromSavants: function(){
    //do stuff here with arr
    if(stuff happens here){
      this.doNextFunction(arr)
    }
  },
  doNextFunction: function(arr){
    //do stuff
  }
}

This executes the getTracksFromSavants function fine, but once it gets to the if it says that doNextFunction is not defined.

On the flip side, if I write my Cron like this:

    var job = new CronJob('00 51 17 * * 0-6', savantJS.getTracksFromSavants(), null, true, 'America/Los_Angeles');

It executes the callbacks in getTracksFromSavants() fine. However the issue is that when I use the () in the CronJob object it executes that function every time the server is started when it should only be executing it at 5:51pm.

Any ideas?

Answer 1

var obj = {
  f: function() {
    return this;
  }
}

obj.f() // returns obj
var ff = obj.f;
ff(); // does not return obj - `ff` does not know the function came from `f`.
ff = ff.bind(obj);
ff(); // returns obj again

In JavaScript, calling a method using a dot notation will set this . calling a method without the dot does not. When you pass savantJS.getTracksFromSavants into a function, it becomes some parameter; when that parameter gets called, it does not know it has any connection to savantJS . Unless you bind the receiver to it.

Thus, you want to wrap your method call inside a normal function (that does not depend on being invoked as a method):

var job = new CronJob('00 51 17 * * 0-6', function() {
  savantJS.getTracksFromSavants();
}, null, true, 'America/Los_Angeles');

Or you can bind the function to a receiver (which basically does the same thing):

var job = new CronJob('00 51 17 * * 0-6', 
  savantJS.getTracksFromSavants.bind(savantJS),
  null, true, 'America/Los_Angeles');