Node.js Class Module System

Question

I'm trying to create a module/class in node.js to measure asynchronous execution time but do not understand whats wrong with it. I created the following class "Measure.js"

var Measure = module.exports = function(param_timeout, param_cb) {

    this.timeout = param_timeout;
    this.cb = param_cb;
}

Measure.prototype = {
    startDate: "0",
    timeout:"0",
    cb:null,

    start : function() {
        this.startDate = new Date();
        console.log('started');
    },

    stop : function() {
        var stopDate = new Date();
        this.cb(null,(stopDate-this.startDate));
    }
}

I use it with the following code:

var Measure = require('./Measure.js');
measure1 = new Measure(100,function(err,result){console.log('result: ' + result)});
measure1.start();
//do something 
measure1.stop();

and it works just fine. However, if I try this:

var Measure = require('./Measure.js');
measure1 = new Measure(100,function(err,result){console.log('result: ' + result)});
measure1.start();
//do something 
setTimeout(measure1.stop,100);

it doesn't work and throws a TypeError:

TypeError: Object #<Object> has no method 'cb'

Whats wrong with my code?

Answer 1

When you directly call object's method, this inside the method referred to your object, but when you try to use it as argument, this will referred to global object ( global or window ).

In your case better to replace

setTimeout(measure1.stop,100);

with

setTimeout(function() { measure1.stop(); }, 100);

More about this behavior: http://bonsaiden.github.com/JavaScript-Garden/#function.this

Answer 2

The dreaded timeout-switch-context bug strikes again! You see what you see because this object in function called by setTimeout is not measure1 - it's global (= window , when this script is executed in browser). Quoting the MDN :

Code executed by setTimeout() is run in a separate execution context to the function from which it was called. As a consequence, the this keyword for the called function will be set to the window (or global ) object, it will not be the same as the this value for the function that called setTimeout .

It's quite easy to check, by the way:

stop: function() {
  var stopDate = new Date();
  if (! this.cb) { 
    console.log(this); 
  } // *Window*/_display...
}

... and to fix as well:

setTimeout(function() { 
  measure1.stop(); 
}, 100);