How do I understand this default statement in the Redux shopping cart example?

Question

Here's the statement:

default:
      const { productId } = action
      if (productId) {
        return {
          ...state,
          [productId]: products(state[productId], action)
        }
      }
      return state

Source: https://codesandbox.io/s/github/reactjs/redux/tree/master/examples/shopping-cart (src/reducers/products/ - line 26)

Here's what I understand so far:

Line 26: If case RECIEVE_PRODUCTS is not met, execute what comes after the double dots.

Line 27: const productId is set equal to action.productId carried over by the addToCart action.

Line 28: If action.productId exists, execute the statement between the curly brackets.

Line 29 & 30: Return the unchanged properties of state.

Line 31: ?

Line 32-34: Otherwise, return state unchanged.

Answer 1

[productId]: products(state[productId], action) is assigning the result of products(state[productId], action) to a key with the value of productId . Its easier to explain with an example:

 const a = 'foo'; const b = 'bar'; const obj = { [a]: a, b: b } console.log(obj);