Javascript，instanceof函数的行为

Question

I'm creating a Node.js application. I have a project with this structure:

[Project Folder]
  |
  |---[plc]
  |     |--- plc.js
  |     |--- scheduler.js
  |
  |---[source]
  |     |--- source.js
  |
  |---[test]
        |--- test.js

The files plc.js , scheduler.js and source.js are "objects", they require other objects and, at the end of file, have an "export" of the object. In particular the file plc.js has an weird behavior. First the code:

var mod_pollist     = require('./polling_list.js');     // Polling list.
var mod_operation   = require('./operation.js');        // Single operation.
var mod_scheduler   = require('./scheduler.js');        // Scheduler object.
var mod_events      = require('events');            // Event emitter
var mod_util        = require('../util.js');        // Util functions

function plc(comm_driver){
    var self = this;
    // Other variables are set here
}

// Other functions written as plc.prototype.something = function(parameters){...}

module.exports = plc;

Now the weird behavior: all the other files have at the top of the file the code for importing the plc.js ( var mod_plc = require('../plc/plc.js'); or var mod_plc = require('./plc.js'); for the scheduler) but only in test.js it works correctly, infact if I write

if(PLC instanceof mod_plc)
    console.log('yes');

in the file test.js I can find a 'yes' on the console, if I write the same code in the other files I obtain an error:

if(PLC instanceof mod_plc)
                  ^
TypeError: Expecting a function in instanceof check, but got #<Object>
    at Object.<anonymous> (C:\Users\Massimo\workspace\Scada\plc\scheduler.js:16:
19)

A "temporary solution" could be

if(PLC instanceof mod_plc.constructor)
    console.log('yes');

but I don't think is the real solution because with all other objects (I've more than 20 files written like plc.js ) this problem doesn't exist.

Any suggestion? Do you need more informations? Thanks

Answer 1

To summarise my comments from above:

Given that:

• the TypeError tells you mod_plc is an Object (not a constructor function); and
• using mod_plc.constructor gives you the expected behaviour;

it looks like your PLC variable has somewhere been assigned an instance of mod_plc (hence is no longer a reference to the expected constructor function).