Shallow `Object.assign` fill-in

assign.js

const assign = (obj, ...props) => props.reduce((acc, prop) => {
    for (let i in prop) acc[i] = prop[i]
    return acc
  }, obj)


// Usage

const state = {
  todos: [
    { text: 'one' }
  ],
  current: ''
}

const newState = assign(
  {},
  state,
  {
    todos: state.todos.concat({ text: 'two' })
  }
)