Using Object Spread Operator

Since one of the core tenets of Redux is to never mutate state, you'll often find yourself using Object.assign() to create copies of objects with new or updated values. For example, in the todoApp below Object.assign() is used to return a new state object with an updated visibilityFilter property:

function todoApp(state = initialState, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    default:
      return state
  }
}

While effective, using Object.assign() can quickly make simple reducers difficult to read given its rather verbose syntax.

An alternative approach is to use the object spread syntax proposed for the next versions of JavaScript which lets you use the spread (...) operator to copy enumerable properties from one object to another in a more succinct way. The object spread operator is conceptually similar to the ES6 array spread operator. We can simplify the todoApp example above by using the object spread syntax:

function todoApp(state = initialState, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return { ...state, visibilityFilter: action.filter }
    default:
      return state
  }
}

The advantage of using the object spread syntax becomes more apparent when you're composing complex objects. Below getAddedIds maps an array of id values to an array of objects with values returned from getProduct and getQuantity.

return getAddedIds(state.cart).map(id => Object.assign(
  {},
  getProduct(state.products, id),
  {
    quantity: getQuantity(state.cart, id)
  }
))

Object spread lets us simplify the above map call to:

return getAddedIds(state.cart).map(id => ({
  ...getProduct(state.products, id),
  quantity: getQuantity(state.cart, id)
}))

Since the object spread syntax is still a Stage 3 proposal for ECMAScript you'll need to use a transpiler such as Babel to use it in production. You can use your existing es2015 preset, install babel-plugin-transform-object-rest-spread and add it individually to the plugins array in your .babelrc.

{
  "presets": ["es2015"],
  "plugins": ["transform-object-rest-spread"]
}

Note that this is still an experimental language feature proposal so it may change in the future. Nevertheless some large projects such as React Native already use it extensively so it is safe to say that there will be a good automated migration path if it changes.