JavaScript function doesn't work

Question

I'm using the following JavaScript code:

var emp= new Object();
emp["name"]="pooja";
emp["salary"]=725;

emp["paycheck"]=function()
 {
  var monthly=this["salary"]/12;
  alert(this["name"]+":"+monthly);
 };

emp["paycheck"]();       --work properly 
document.write("<br/>");
var f=emp["paycheck"];    --dosen't work
f();

f() have to get reference on emp["paycheck"] function and display a suitable answer. but insted i get NaN.

As i understood f() dosen't see the property of emp object("name" and "salary"). My question is why f() dosen't see the properties?

pimvdb · Accepted Answer · 2012-02-04 22:28:23Z

4

You refer to salary as this["salary"]. If you store the function and call it later, the this value is lost. It is only bound to the object if you directly call it, as in emp.paycheck(). You could pass the this value explicitly, though:

f.call(emp);

But you might rather want to refer to salary in the function as emp["salary"], since that will always work.

Note that instead of foo["bar"] you can use foo.bar, and that the new Object() part can just be:

var emp = {
  name: "pooja",

  salary: 725,

  paycheck: function() {
    ...
  }
};

edited Feb 4, 2012 at 22:28

answered Feb 4, 2012 at 18:37

pimvdb

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1 Comment

thenetimp Over a year ago

This method is the far more commonly accepted way to build an object.

JaredPar · Accepted Answer · 2012-02-04 18:37:50Z

2

The reason why is you are calling the function without a this parameter. You need to use apply to pass it an explicit this on which it can call the name and salary properties.

f.apply(emp);

answered Feb 4, 2012 at 18:37

JaredPar

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Comments

Dezigo · Accepted Answer · 2012-02-04 18:40:47Z

1

you reference on a function, not on all object.

f = emp;   
f.paycheck();

answered Feb 4, 2012 at 18:40

Dezigo

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Comments

Community · Accepted Answer · 2017-05-23 12:11:23Z

1

You didn't copy those properties to f. There are, unfortunately, no native ways to do this in JavaScript. See How do I correctly clone a JavaScript object? or What is the most efficient way to deep clone an object in JavaScript?, look at jQuery's extend(), or google "javascript deep copy object" or "javascript clone object".

edited May 23, 2017 at 12:11

CommunityBot

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answered Feb 4, 2012 at 18:44

nshew13

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Comments

Kos · Accepted Answer · 2012-02-04 18:44:28Z

1

Others have explained what's going on and how you don't pass this.

However, if you want to do something like this (a custom from Python perhaps?), you can make a helper function:

function makeDelegate(obj, fun) {
    return function() {
        fun.apply(obj, arguments);
    };
}

Then use it like this:

var f = makeDelegate(emp, emp["paycheck"]);
f();

answered Feb 4, 2012 at 18:44

Kos

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