Spread Operator not working for Redux/ES6 based sample

Question

I am trying to understand Redux online tutorials that are posted by Dan Abramov. At present I am on the following sample:

Reducer composition with Arrays

Here is my practice code following the above sample:

// Individual TODO Reducer
const todoReducer = (state, action) => {
    switch(action.type) {
    case 'ADD_TODO':
        return {
            id: action.id,
            text: action.text,
            completed: false
          };
    case 'TOGGLE_TODO':
        if (state.id != action.id) return state;

      // This not working
      /*
      return {
        ...state,
        completed: !state.completed
      };
      */

      //This works
      var newState = {id: state.id, text: state.text, completed: !state.completed};
      return newState;
    default:
        return state;
  }
};

//TODOS Reducer
const todos = (state = [], action) => {
        switch(action.type) {
        case 'ADD_TODO':
       return [
          ...state,
          todoReducer(null, action)
       ];
       case 'TOGGLE_TODO':
        return state.map(t => todoReducer(t, action));
      default:
        return state;
    }
};

//Test 1
const testAddTodo = () => {
  const stateBefore = [];

  const action = {
      type: 'ADD_TODO',
      id: 0,
      text: 'Learn Redux'
  };

  const stateAfter = [{
     id: 0,
     text: "Learn Redux",
     completed: false
  }];

  //Freeze
  deepFreeze(stateBefore);
  deepFreeze(action);

  // Test
  expect(
     todos(stateBefore, action)
  ).toEqual(stateAfter);
};

//Test 2
const testToggleTodo = () => {
  const stateBefore = [{id: 0,
     text: "Learn Redux",
     completed: false
  }, {
    id: 1,
    text: "Go Shopping",
    completed: false
  }];

  const action = {
      type: 'TOGGLE_TODO',
      id: 1
  };

  const stateAfter = [{
     id: 0,
     text: "Learn Redux",
     completed: false
  }, {
    id: 1,
    text: "Go Shopping",
    completed: true
  }];

  //Freeze
  deepFreeze(stateBefore);
  deepFreeze(action);

  // Expect
  expect(
     todos(stateBefore, action)
  ).toEqual(stateAfter);
};

testAddTodo();
testToggleTodo();
console.log("All tests passed");

Issue is, within the todoReducer function, following syntax is not working:

return {
        ...state,
        completed: !state.completed
      };

I am using Firefox version 44.0 and it shows me following error in console:

Invalid property id

Now I guess my current version of Firefox must support Spread operator. If anyway it does not, is there any way to add some standalone Polyfill to support this syntax?

Here is also the JSFiddle

Answer 1

The object spread syntax is not supported in most browsers at the minute. It's proposed for addition in ES7 (aka ES2016). As far as I know there's no way to polyfill it, as it uses a new syntax rather than just being a function call.

You have two options in the meantime.

1) Use Object.assign to create an updated version of the object, like so:

Object.assign({}, state, {
  completed: !state.completed
});

Although this will also need to be polyfilled in most browsers - a good example one is available on MDN, or you can use a third party library's version, like the one in lodash.

2) Use transpiling tools like Babel, which allow you to write your code with newer syntax and then convert it to a version that works in all browsers.

Answer 2

If anyone using Babel is still having trouble, this feature may not be available with Babel out of the box, you may need to add a plugin: http://babeljs.io/docs/plugins/transform-object-rest-spread/

then update .babelrc with

"plugins": ["transform-object-rest-spread"]

Answer 3

You can not polyfill syntax. You need to use something like babel to compile to an older version of JavaScript if you wish to execute in current browsers.

https://babeljs.io/