Trying to add items to a list

formdata.js

const choo = require('choo')                                                                                                                                                                    
const app = choo()                                                                                                                                                                              
                                                                                                                                                                                                
function view (params, state, send) {                                                                                                                                                           
      return choo.view`                                                                                                                                                                         
          <main>                                                                                                                                                                                
              <h1>${state.title}</h1>                                                                                                                                                           
              <ul>                                                                                                                                                                              
                ${state.theList.map(function (item) {                                                                                                                                           
                  return choo.view`<li>${item}</li>`                                                                                                                                            
                })}                                                                                                                                                                             
              </ul>                                                                                                                                                                             
              <form onsubmit=${onSubmit}>                                                                                                                                                       
              New: <input type="text" name="new_item" value="foo" autofocus>                                                                                                                    
                   <input type="submit" value="Submit">                                                                                                                                         
              </form>                                                                                                                                                                           
          </main>                                                                                                                                                                               
          `                                                                                                                                                                                     
    function onSubmit (event) {                                                                                                                                                                 
        send('addItem', { data: new FormData(event.target) });                                                                                                                                                
        event.preventDefault()                                                                                                                                                                  
    }                                                                                                                                                                                           
                                                                                                                                                                                                
}                                                                                                                                                                                               
                                                                                                                                                                                                
var newlist = ['lion', 'tiger', 'bear' ];                                                                                                                                                       
app.model({                                                                                                                                                                                     
    state: {                                                                                                                                                                                    
        title: 'Some title',                                                                                                                                                                    
        theList: ['item 1', 'item 2', 'item 3', 'item 4' ]                                                                                                                                      
    },                                                                                                                                                                                          
    reducers: {                                                                                                                                                                                 
        addItem: function(action,state) {                                                                                                                                                       
            console.log('action=', action.data, ' state=', state);                                                                                                                              
            return { theList: state.theList.push(action.value) }                                                                                                                                
            }                                                                                                                                                                                   
    }                                                                                                                                                                                           
});   

app.router((route) => [                                                                                                                                                                         
    route('/', view)                                                                                                                                                                            
]);                                                                                                                                                                                             
                                                                                                                                                                                                
const tree = app.start();                                                                                                                                                                       
document.body.appendChild(tree);