JavaScript (Underscore.js) Functionality Extension

Disclaimer: (1) My back platform is in Java / C #, and I just started my deep dive in JavaScript last week. (2) I know the _.mixin () method.

This is not a critical project, but rather, I am having problems with overlaying object inheritance in Javascript.

For example, I tried to increase underscore.js using the _.keyFilter function (I understand that I can use the map to achieve something similar) to return a list of keys that satisfy the evaluation function. I can achieve the result using the _.mixin () method:

Besides dropping the function directly into the source code, _.mixin ()

 _.mixin({ filterKey : function(obj, iterator, context) {
    var results = [];
    if (obj == null) return results;
    _.each(obj, function(value, index, list) {
        if (iterator.call(context, value, index, list)){
            results[results.length] = index;
        }
    });
    return results;
}});

However, I'm not sure I understand why I cannot just increase the underline with the following in the script file:

_.keyFilter = function(obj, iterator, context) {
        var results = [];
        if (obj == null) return results;
        each(obj, function(value, index, list) {
            if (iterator.call(context, value, index, list)) results[results.length] = index;
        });
        return results;
};

, :

_.chain(myList).keyFilter(evalFunction);

:

'Uncaught TypeError: Object [object Object] has no method 'keyFilter' 

, , underscore.js.

, ( ) script:

var keyFilter = _.prototype.keyFilter = function(obj, iterator, context) {
        var results = [];
        if (obj == null) return results;
        each(obj, function(value, index, list) {
            if (iterator.call(context, value, index, list)) results[results.length] = index;
        });
        return results;
};

. ( , var script, ).

.