robust-user-interfaces-with-state-machines - for medium

Promise naive implementation

function makePromiseMachine(params) {  
  let state = {  
    control: 'pending',  
    // ... other pieces of state  
  };  
  
  return function (event) {  
    const control = state.control;  
    let output;  
  
    switch (event){  
      case 'approve':  
        switch (control) {  
          case 'pending':  
            // update state, update output  
            state.control = 'approved'  
            output = ...  
            break;  
  
          default:  
            break;  
        }  
        break;  
  
      case 'reject':  
        switch (control) {  
          case 'pending':  
            // update state, update output  
            state.control = 'rejected'  
            output = ...  
            break;  
  
          case 'approved':  
            // update state, update output  
            state.control = 'rejected'  
            output = ...  
            break;  
  
          default:  
            break;  
        }  
        break;  
  
      case 'pend':  
        (...)
        break;  
  
      default:  
        break;  
    }  
  
    return output  
  }  
}