Using for-in loop inside object to access its member

Question

function Cons()
    {
        this.a = "variable a";
        this.callme = function callme()
        {
            var str = "";
                for(var x in this)
                {
                    str += this[x] + " ";
                }
            return str;
        }
    }

    var obj = new Cons();

    obj.c = "variable 2";

    var fin = obj.callme();
    document.write(fin);

I want to have a function inside the object so that when it is called it can return a string consisting of the values of every member. Here in this case a and c. Now what happens, everything inside the function i mean the code is printed inside the browser instead of just returning the value of str.

What i understood is that this["callme"] part in the for-in loop returns the whole code as its also a variable. So how to solve this problem.

I am new to javascript please help me.

What have you researched/tried so far ? I don't see any attempt of looping in the code you posted... — Laurent S.
– Laurent S., Commented Nov 12, 2013 at 17:39
there is a for loop inside the callme function that creates all the problem but thats what i was trying to achieve — Arin Chakraborty
– Arin Chakraborty, Commented Nov 12, 2013 at 17:41
you can skip functions when you loop. if this is what you want. — akonsu
– akonsu, Commented Nov 12, 2013 at 17:42

Tibos · Accepted Answer · 2013-11-12 17:44:37Z

2

There are several ways to solve this:

1) Remove callme from the for..in :

for(var x in this) {
  if (x !== 'callme') {
    str += this[x] + " ";
  }
}

2) Declare callme as a non-enumerable property:

function Cons() {
    Object.defineProperty('callme', {
      enumerable : false,
      value : function callme() {
        var str = "";
            for(var x in this)
            {
                str += this[x] + " ";
            }
        return str;
      }
    });
}

3) Change the toString method of callme to return nothing:

function Cons() {
    this.a = "variable a";
    this.callme = function callme() {
        var str = "";
            for(var x in this)
            {
                str += this[x] + " ";
            }
        return str;
    }
    this.callme.toString = function(){ return '';};
}

The simplest solution is the first one, but the others are too interesting to pass.

answered Nov 12, 2013 at 17:44

Tibos

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2 Comments

Arin Chakraborty Over a year ago

Thank you for helping. I really need to study more to understand the last two as i said i am new to js. But didn't the 1st one will only just skip the callme function instead of skipping all the other functions inside the object?

Tibos Over a year ago

Yes. @matewka gave you a good answer on skipping all the functions, not just callme.

matewka · Accepted Answer · 2013-11-12 17:43:18Z

1

If you want to avoid the function body being printed, check if the property is a function and print only its name:

this.callme = function callme()
    {
        var str = "";
            for(var x in this)
            {
                if ('function' === typeof this[x]) {
                    str += x + " ";
                } else {
                    str += this[x] + " ";
                }
            }
        return str;
    }

answered Nov 12, 2013 at 17:43

matewka

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3 Comments

Arin Chakraborty Over a year ago

Hey thanks a lot. Thanks a lot. This is what i was looking for. Can you please tell me a good guide from where can i learn more about functions and objects of javascript.

Tibos Over a year ago

Mozilla Developer Network (MDN) is THE best place to study things about JavaScript and frontend stuff in general.

matewka Over a year ago

@Tibos, + for you. Here's the link MDN. As an addition - try to avoid w3schools as they're no longer a w3 branch and sometimes provide some misleading information.

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