JavaScript - Arrays - indexOf

javascript-arrays

// JavaScript - Array.prototype.indexOf()

The indexOf() method returns the first index at which a given element can be 
found in the array, or -1 if it is not present.

arr.indexOf(searchElement)
arr.indexOf(searchElement, fromIndex)

searchElement: Element to locate in the array.
fromIndex: Optional.  The index to start the search at. If the index is greater 
  than or equal to the array's length, -1 is returned, which means the array 
  will not be searched. If the provided index value is a negative number, it is 
  taken as the offset from the end of the array. Note: if the provided index is 
  negative, the array is still searched from front to back. If the calculated 
  index is less than 0, then the whole array will be searched. Default: 0 
  (entire array is searched).

The includes() method retusn the first index of the leemnt in the array, or
-1 if not found.

var array = [2, 9, 9];
array.indexOf(2);     // 0
array.indexOf(7);     // -1
array.indexOf(9, 2);  // 2
array.indexOf(2, -1); // -1
array.indexOf(2, -3); // 0