mixin.js

var human = { race: 'human' };
var person = { name: 'Jim', age: 26 };

function mixin(target, source, props) {
  props = props || Object.keys(source);

  props.forEach(function(prop) {
    target[prop] = source[prop];
  });

  return target;
}

// => { name: 'Jim', age: 26, race: 'human' }
console.log(mixin(person, human));