Assuming that the value passed to flattenKeys() is not recursive, this function will recursively flatten all of the underlying keys for objects and arrays.

flattenKeys.js

var flattenKeys;
(function() {
  function recurseFlat(value, path) {
    var iterType = Array.isArray(value)
      ? 'A'
      : (value && 'object' === typeof value);
    if (iterType) {
      var keys = iterType === 'A'
        ? value.map(function(v,i){return i})
        : Object.keys(value);
      return keys.reduce(function(returnValue, key) {
        var innerValue = value[key];
        var newPath = path.concat([encodeKey(key)]);
        if (innerValue && 'object' === typeof innerValue) {
          returnValue = Object.assign(returnValue, recurseFlat(innerValue, newPath));
        }
        else {
          returnValue[newPath.join('.')] = innerValue;
        }
        return returnValue;
      }, {});
    }
    return value;
  }

  function encodeKey(key) {
    return (key + "").replace(/[\\\.]/g, '\\$&');
  }

  flattenKeys = function(value) {
    return recurseFlat(value, []);
  };
})();

~example.js

// HIDE \\
console.load('local://flattenKeys.js');

//\\
flattenKeys({
  name: {
    first: "John",
    last: "Jacob"
  },
  age: 56,
  gender: "Male",
  relatives: [
    {
      name: {
        first: "Tina",
        last: "Jacobs"
      }
    },
    {
      name: {
        first: "Sarah",
        last: "Connor"
      }
    }
  ]
});

Testing flatten() function:
https://www.yourjs.com/console/?gist=77c370d62724935f2473c9b70fbe2ecc&file=~example.js&skipIntro=1