Assign value by path.

assign.js

function assign(obj, path, fn) {

    if (obj === null || typeof(obj) === 'undefined')
        return obj;

    var arr = path.split('.');
    var model = obj[arr[0]];

    for (var i = 1; i < arr.length - 1; i++)
        model = model[arr[i]];

    model[arr[arr.length - 1]] = typeof (fn) === 'function' ? fn(model[arr[arr.length - 1]]) : fn;
    return obj;
}

// ==============================
// EXAMPLE
// ==============================

var obj = { user: { address: { street: 'Viestova 12' } }, name: 'Peter Širka' };

console.log(obj);

assign(obj, 'user.address.street', 'Teplicka 19');

console.log(obj);

// OR

assign(obj, 'user.name', function(oldvalue) {
  return 'Janko Hraško';
});

console.log(obj);