Search within an Array

You've used indexOf to search for a substring within a string. Arrays also have an indexOf method, and it works the same way:

let ingredients = ["flour", "sugar", "eggs", "butter"];
ingredients.indexOf("eggs"); // 2
ingredients.indexOf("salt"); // -1

indexOf returns the first index where the search value is found. If you want to find the last index where the search value is found, you can use lastIndexOf:

let cards = ["hearts", "diamonds", "clubs", "spades", "hearts"];
cards.indexOf("hearts"); // 0
cards.lastIndexOf("hearts"); // 4

Tests

Write a function called exactlyOnce that takes an array and a value. It should return true if the value appears in the array exactly once, but false if it appears more than once or not at all.