code: function PromiseSet(promise, status, value) {
    SET_PRIVATE(promise, promiseStateSymbol, status);
    SET_PRIVATE(promise, promiseResultSymbol, value);
  
    // There are 3 possible states for the resolve, reject symbols when we add
    // a new callback --
    // 1) UNDEFINED -- This is the zero state where there is no callback
    // registered. When we see this state, we directly attach the callbacks to
    // the symbol.
    // 2) !IS_ARRAY -- There is a single callback directly attached to the
    // symbols. We need to create a new array to store additional callbacks.
    // 3) IS_ARRAY -- There are multiple callbacks already registered,
    // therefore we can just push the new callback to the existing array.
    SET_PRIVATE(promise, promiseFulfillReactionsSymbol, UNDEFINED);
    SET_PRIVATE(promise, promiseRejectReactionsSymbol, UNDEFINED);
  
    // There are 2 possible states for this symbol --
    // 1) UNDEFINED -- This is the zero state, no deferred object is
    // attached to this symbol. When we want to add a new deferred we
    // directly attach it to this symbol.
    // 2) symbol with attached deferred object -- New deferred objects
    // are not attached to this symbol, but instead they are directly
    // attached to the resolve, reject callback arrays. At this point,
    // the deferred symbol's state is stale, and the deferreds should be
    // read from the reject, resolve callbacks.
    SET_PRIVATE(promise, promiseDeferredReactionsSymbol, UNDEFINED);
  
    return promise;
  }
docs: Core functionality.