// code (that's all):

var mjp = (p, o, n) => p.split(/\.|\[|\]/).filter((x) => !!x).reduce((a, c, i, l) => l.length-i <= n ? a : a[c], o);

// usage:

let obj = {
  "x": 1,
  "y": [
    [
      {
        "z": 4
      }
    ],
    [
      {
        "z": 6
      }
    ]
  ]
};

let path = 'y[0][0].z';

let nestedValue = mjp(path, obj); // 4

// You can set a third argument to get the nth parent from the path
let nestedValueFirstParent = mjp(path, obj, 1); // {z: 4}
let nestedValueSecondParent = mjp(path, obj, 2); // [{"z":4}]