gistfile1.js

let defaults  = {},
    overrides = {};

function foo(bar, options) {
    // YUI
    options = Y.merge(defaults, options, overrides);
    
    // Underscore
    options = _.extend({}, defaults, options, overrides);
    
    // ES6
    options = [defaults, options, overrides].reduce(Object.assign, {});
}

// ES6 YUI `Object.assign()` sugar.
Y.assign = function (target, ...sources) {
    return sources.reduce(Object.assign, target);
};

// ES6 back-compat `Y.merge()`.
Y.merge = function (...sources) {
    return Y.assign({}, ...sources);
}