Trouble passing arguments through an initialiser function in javascript

Question

I am having trouble passing arguments through a new object when using an initialiser function. Consider my following examples that I want to be able to create an object that returns an array. Object Ex1 does this fine:

Ex1 = function() {

    myVar = [];
    myVar = Array.apply( myVar, arguments );
    return myVar;

};

ex1 = new Ex1( 'red', 'green', 'blue' );
console.log( ex1);
/* ["red", "green", "blue"] */

However, I want to us an initialiser to keep my code clean. Object Ex2 shows my unexpected result:

Ex2 = function() {

    init = function() {
        myVar = [];
        myVar = Array.apply( myVar, arguments );
        return myVar;
    };

    return init( arguments );

};

ex2 = new Ex2( 'red', 'green', 'blue' );
console.log( ex2 );
/* [[object Arguments] {
   0: "red",
   1: "green",
   2: "blue"
   }] */

As you can see from the log the result is not a clean array.

How do I return an array when creating a new object when using an initialiser function and passing arguments to it?

You need to learn to use local variables…

Bergi
– Bergi

2013-10-13 15:48:22 +00:00
Commented Oct 13, 2013 at 15:48 — Bergi
– Bergi, Commented Oct 13, 2013 at 15:48
A very helpful comment...

McShaman
– McShaman

2013-10-13 19:53:32 +00:00
Commented Oct 13, 2013 at 19:53 — McShaman
– McShaman, Commented Oct 13, 2013 at 19:53

grape_mao · Accepted Answer · 2013-10-13 16:55:40Z

1

This problem is not hard to understand. In a function, arguments is an array-like object which contains all the arguments passed in. In your second example, you passed an array as argument to init function, so arguments in init function is a 2D array.

Ex2 = function() {   
    init = function() {
        myVar = [];
        myVar = Array.apply( myVar, arguments );//arguments: [["red", "green", "blue"]]
        return myVar;
    };

    return init( arguments );//arguments: ["red", "green", "blue"]  
};

You could use a parameter for example:

Ex2 = function() {
    var init = function(argu) {
        return Array.apply( 0, argu );//argu: ["red", "green", "blue"]
    };
    return init( arguments );
};
ex2 = new Ex2( 'red', 'green', 'blue' );
console.log( ex2 );//["red", "green", "blue"]

Another thing is declaring a variable without var will put it in global scope.

edited Oct 13, 2013 at 16:55

answered Oct 13, 2013 at 10:07

grape_mao

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1 Comment

McShaman Over a year ago

Ah I see now. By passing the arguments object as an argument it is getting wrapped within another arguments object. Thanks... Your post and a good nights sleep has helped me understand. :)

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