JavaScript – cannot set property of undefined

you never set d[a] to any value.

Because of this, d[a] evaluates to undefined, and you can’t set properties on undefined.

If you add d[a] = {} right after d = {} things should work as expected.

Alternatively, you could use an object initializer:

d[a] = {
    greetings: b,
    data: c
};

Or you could set all the properties of d in an anonymous function instance:

d = new function () {
    this[a] = {
        greetings: b,
        data: c
    };
};

If you’re in an environment that supports ES2015 features, you can use computed property names:

d = {
  [a]: {
    greetings: b,
    data: c
  }
};