Here’s the original snippet from the updateAverageRating function which uses Array.reduce to count all the review ratings.
const total = location.reviews.reduce( (accumulator, rating) => {
return accumulator + rating;
});My code had two problems which left me puzzled in the debugger for a bit.
- Why was rating a MongoDb document instead of the rating from that document?
- Why was accumulator also a MongoDb document instead of a numeric counter?
When writing the callback function, I failed to notice the use of the destructuring curly brace syntax:
{ rating } This is a lovely piece of JS syntax which allows you to unpack values from objects in a really useful way. In this example, we are selecting the rating property of the document for use in the reduce arrow function.
The issue with the accumulator was due to a missing optional parameter to the reduce function: initialValue.
location.reviews.reduce( <callback>, 0);
Without the initialValue parameter, “the first element in the array will be used as the initial accumulator value”