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https://www.w3schools.com/jsref/jsref_obj_array.asp
The indexOf() also works with arrays. You can use it to determine if something is found within an array. If it is NOT found, it will return a -1. "0" is actually the very first position (zero based) so it's possible o is return as a positive test.
// The result of a will be “2” meaning that "Apple" is located at position 2 in the array.
let fruits = ["Banana", "Orange", "Apple", "Mango"];
let a = fruits.indexOf("Apple");
document.getElementById("demo").innerHTML = a;