nested-json.js

var setNestedJSON = function(on, path, value) {
  if (!(path instanceof Array)) {
    path = path.split('\.');
  }

  if (path && path.length > 0) {
    var current = on;
    var key = null;

    while((key = path.shift()) != null) {
      // make sure key exists at the current level
      if (!current[key]) {
        current[key] = {};
      }
      if (path.length == 0) {
        // last level, set value
        current[key] = value;
        return true;
      }
      // advance one level
      current = current[key];
    }
  }

  return false;
}

var a = {'a':'b', 'b': {'c':'d', 'e':'f'}};

console.log(a);

setNestedJSON(a, 'b.c', 42);

console.log(a);

setNestedJSON(a, 'b.d', 42.5);

console.log(a);

setNestedJSON(a, 'c', {'foo':'bar'});

console.log(a);

setNestedJSON(a, 'c.baz', 'foobaz');

console.log(a);

setNestedJSON(a, ['c', 'baz'], '1234');

console.log(a);