Dynamically instantiate class in javascript

Question

I checked out this reference link and ended up using Matthew's solution, as it works for me.

var factory ={};
factory.Tree = function(arg1,arg2,arg3,arg4){
    console.log(arg1+""+arg2+""+arg3+""+arg4); 
}
function instantiate(classname){
    return new (function(a){ return factory[classname].apply(this,a);})(Array.prototype.splice.call(arguments,1));
    // also is this ^^^ a good practice? instead of declaring the temp function beforehand
    // function t(a) {return factory[classname].apply(this,a);}
    // return new t(Array.prototype.splice.call(arguments,1));
}
var newObj = instantiate("Tree",1,2,3,4); // this prints 1234 --> works

Though, I'm not sure why using user123444555621's solution only works if I pass in "arguments" (that is everything including "classname"):

function instantiate(classname){
    return new (Function.prototype.bind.apply(factory[classname], arguments));
}
var newObj = instantiate("Tree",1,2,3,4); // this prints 1234 --> works

but if I slice "arguments" and remove "classname", then pass in the result array, it does not work as expected:

function instantiate(classname){
    var args = Array.prototype.splice.call(arguments,1); 
        // ^^ I checked and this prints 1,2,3,4 as well
    return new (Function.prototype.bind.apply(factory[classname], args));
}
var newObj = instantiate("Tree",1,2,3,4); // this prints 234undefined

I'm not sure why but somehow it seems like the args array is sliced (again) and removes its first element (1 in this case).

Could someone offer any insights? Thanks

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Did you use the right array function slice vs splice ?

Array.prototype.slice() - Creates a new array from elements of an existing array. It does not modify the original array.

Array.prototype.splice() – Deletes and/or inserts elements in an array. Unlike slice(), the splice() method modifies the original array and returns a new array. The splice() method takes three arguments.

edited Jun 20, 2020 at 9:12

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answered Apr 16, 2016 at 7:30

shershen

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Comments

gurvinder372 · Accepted Answer · 2016-04-16 07:49:56Z

0

Your problem is this line

var args = Array.prototype.splice.call(arguments,1);

You have essentially removed the first item from the arguments while turning it into array args

simply replace 1 by 0

var args = Array.prototype.splice.call(arguments,0);

DEMO

var factory ={};
factory.Tree = function(arg1,arg2,arg3,arg4){
    console.log(arg1+""+arg2+""+arg3+""+arg4); 
}
function instantiate(classname){
    var args = Array.prototype.splice.call(arguments,0); 
    return new (Function.prototype.bind.apply(factory[classname], args));
}
instantiate("Tree",1,2,3,4);

answered Apr 16, 2016 at 7:49

gurvinder372

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1 Comment

rustyengineer Over a year ago

Thanks. Correct me if I'm wrong but aren't we supposed to remove the first argument and return the remaining as an array? In this case, remove "Tree", and pass in [1,2,3,4] to the constructor of Tree? If not removing anything from arguments, then maybe I could just pass it directly to apply() (just not sure why it works)

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