Extend massactions.js

Looks like answered on github. copy/past here

You can try 'mixins' feature, all you need is to create requireJS module with extended method like this:

default_mixin.js:

define(function () {
    'use strict';

    var mixin = {

        methodToExtend: function () {
            //...
        }
    };

    return function (target) { // target == Result that Magento_Ui/.../default returns.
        return target.extend(mixin); // new result that all other modules receive 
    };
});

requirejs-config.js:

config: {
    mixins: {
        'My_Module/js/payment/default': {  // Target module
            'My_Module/js/payment/default_mixin': true  // Extender module
        }
    }
}