`Array.prototype.reduce()`, or simply `reduce()`, is the most general of all array operations in JavaScript. You can solve almost any array processing problem using the `reduce` method.
The `reduce` method allows for more general forms of array processing, and it's possible to show that both `filter` and `map` can be derived as special applications of `reduce`. The `reduce` method iterates over each item in an array and returns a single value (i.e. string, number, object, array). This is achieved via a callback function that is called on each iteration.
The callback function accepts four arguments. The first argument is known as the accumulator, which gets assigned the return value of the callback function from the previous iteration, the second is the current element being processed, the third is the index of that element and the fourth is the array upon which `reduce` is called.
In addition to the callback function, `reduce` has an additional parameter which takes an initial value for the accumulator. If this second parameter is not used, then the first iteration is skipped and the second iteration gets passed the first element of the array as the accumulator.
See below for an example using `reduce` on the `users` array to return the sum of all the users' ages. For simplicity, the example only uses the first and second arguments.
The variable `watchList` holds an array of objects with information on several movies. Use `reduce` to find the average IMDB rating of the movies directed by `Christopher Nolan`. Recall from prior challenges how to `filter` data and `map` over it to pull what you need. You may need to create other variables, and return the average rating from `getRating` function. Note that the rating values are saved as strings in the object and need to be converted into numbers before they are used in any mathematical operations.
"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
"Language": "English, Japanese, French",
"Country": "USA, UK",
"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
"Language": "English, Mandarin",
"Country": "USA, UK",
"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
"Language": "English, Urdu, Mandarin",
"Country": "USA, UK",
"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
"Language": "English, Spanish",
"Country": "USA, UK",
"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
"Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
"Language": "English, Japanese, French",
"Country": "USA, UK",
"Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
"Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
"Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
"Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
"Language": "English, Mandarin",
"Country": "USA, UK",
"Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
"Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
"Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
"Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
"Language": "English, Urdu, Mandarin",
"Country": "USA, UK",
"Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
"Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
"Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
"Language": "English, Spanish",
"Country": "USA, UK",
"Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
