Question

If I have a sting such as "AppNamespace.SomeObject.ClassName" and I know that the string represents and actual constructor function, what is the best/recommend way of getting that to use in a statement like var foo = new AppNamespace.SomeObject.ClassName?

I could do:

var s = "AppNamespace.SomeObject.ClassName"
var foo = new (eval(s))

or something like:

var parts = "AppNamespace.SomeObject.ClassName".split(".")
var foo = new (window[parts[0]][parts[1]][parts[2]])

but I’m wondering if there are better solutions that don’t involve eval or having to split the string and loop through its parts. Does anyone have any ideas? If not, based on the two solutions I’ve proposed, what are the pros and cons of each?

Answer 1

You could do…

var obj = new ('AppNamespace.SomeObject.ClassName'
                .split('.')
                .reduce(function(object, key) {
                            return object[key];
                        }, window));

jsFiddle.

On older browsers, you could shim reduce().

This code splits the string by the period (.) and then iterates over each from left to right, starting with window as the start object (change it if it isn’t) and getting each sub property until it arrives with the far right (ClassName in your example).

The new (...) instantiates this object.

You could write it as a reusable function…

var getObjectByString = function(string, baseObject, delimiter) {
    // Only checks for string primitives, but that's OK for this example.
    if (typeof string !== 'string') {
        throw new TypeError('First argument is required and must be a string.');
    }

    baseObject = baseObject || window;
    delimiter = delimiter || '.';

    return string
            .split(delimiter)
            .reduce(function(object, key) {
                      return object[key];
                    }, baseObject));
};