# Reacting to transitions

If a flow transitions from one state to another, you can run a reaction.

To run a reaction, add an object to the when object using the name of the current state, and then, inside of this object, a function with the name of the new state. This function takes three parameters: the flow itself, the event, and a mark object.

Inside of the function, add code that reacts to the transition. To read the flow's state, use the state property of the flow. Once you are done, call the mark.asDone function. If something failed, you may call the mark.asRejected function and provide a reason.

E.g., to run a reaction when a flow transitions from pristine to awaiting-payment, use the following code:

const when = {
  pristine: {
    'awaiting-payment' (flow, event, mark) {
      // ...

      mark.asDone();
    }
  }
};