Dynamically calling an object's private functions

Question

I can't seem to dynamically call the private method add from a public function. It seems that this only accesses the public scope which is why I'm not able to call add. Is there a way to do this?

function test()
{
    var actionCollection = [];

    function add( int1, int2 )
    {
        return int1 + int2;
    }

    this.callFunc = function( testMethodFunction, methodArguments )
    {
        this[testMethodFunction].apply(null, methodArguments);//Method 'add' not found.
    }
}

var t = new test();

alert( t.callFunc( 'add', [1,2] ) );

Plus I'm not exactly sure what null is supposed to do considering you can also use this in the apply argument. Could I also have some clarification on what the first argument of apply is supposed to do? Since this is also related to my original question. Thanks in advance for any help!

[.apply()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply) — Andreas, Aug 14 '13 at 10:45
@Andreas I already read that, I'm still not clear on the purpose of the first argument however. — Tek, Aug 14 '13 at 10:54
The first argument specifies the value which is accessible via `this` in the function. `function foo() { console.log("this.bar = " + this.bar); } foo.apply({ bar: 5 }, null);` will log `this.bar = 5` — Andreas, Aug 14 '13 at 11:13

score 0 · Accepted Answer · answered Aug 14 '13 at 10:43

add is not a part of this. Therefore you cannot use this[testMethodFunction]. If you want to keep privacy, then you can use something like this:

function test() {
    var actionCollection = [];

    var private_methods = {
        add: function( int1, int2 ) {
            return int1 + int2;
        }
    }

    this.callFunc = function( testMethodFunction, methodArguments )
    {
        // note the change here!
        return private_methods[testMethodFunction].apply(null, methodArguments);
    }
}

var t = new test();

alert( t.callFunc( 'add', [1,2] ) );

Great, thanks. Accepted since it was the first correct answer :) — Tek, Aug 14 '13 at 10:56

score 0 · Answer 2 · answered Aug 14 '13 at 10:54

This is because add() is not a property of Test, it is only a local variable in the Test() closure.

Here's a sample code:

function Test()
{
    var _priv = {
        add: function ( int1, int2 )
        {
            console.log(int1, int2);
            return int1 + int2;
        }
    };

    this.callFunc = function( testMethodFunction, methodArguments )
    {
        console.log(_priv);
        return _priv[testMethodFunction].apply(null, methodArguments);
    }
}

var t = new Test();

console.log(t);

console.log( t.callFunc( 'add', [1,2] ) );

Some tips for you:

Do use upper case for Class-like constructs (Test instead of test)
Use log() to your advantage to inspect objects

Dynamically calling an object's private functions

2 Answers2