Polyfill js to extend objects and child objects

example-extend-defaults.js

var defaults = {
 firstName = 'John',
 lastName = 'Doe'
};

var options = {
 firstName = 'Jane'
};

extendDefaults(defaults, options);

console.log(options);

// {firstName = 'Jane', lastName = 'Doe'};

extend-defaults.js

function extendDefaults (a, b) {
  for ( var prop in a ) {
    if ( typeof a[prop] === 'object' && b.hasOwnProperty(prop) ) {
      extendDefaults(a[prop], b[prop]);
      continue;
    }
    if ( !b.hasOwnProperty(prop) ) b[prop] = a[prop];
  }
}