code flow.js

child.js

import * as m from 'mithril'

export const controller = function(props) {
  var self = this
  this.myAction = msg => {
       event.preventDefault()
      // HERE I WANT TO SEND THE 'smg' TO THE 'ctrl.message' ON MainComponent.
      props.message('set to whatever!')
  }
}

export const view = (ctrl, props) => [
  m('h3', 'Option List'),
  m('.list_items', [
    props.items.map(item =>
      m('a', {
        href: '#', 
        onclick: () => ctrl.myAction(item.msg) // could also just do props.message(item.message) directly and drop the controller entirely
      }, item.name)
    )
  ])
]

main.js

import * as m from 'mithril'
import * as ChildComponent from './child'

export const controller = function() {
  this.message = m.prop('An example here')
  this.items = [
    {name: 'test1', msg: 'message test 1'},
    {name: 'test2', msg: 'message test 2'},
    {name: 'test3', msg: 'message test 3'},
    {name: 'test4', msg: 'message test 4'},
    {name: 'test5', msg: 'message test 5'},
  ]
}

export const view = ctrl => [
  m('h1', ctrl.message() ? ctrl.message() : ''),
  m(ChildComponent, { message: ctrl.message, items: ctrl.items })
]