JavaScript's indexOf() function can be used to determine the index of an array element.
The indexOf() function returns -1 if an element is missing from the array or the first index at which the element can be found.
JavaScript Array findIndex()
This method executes a function for each element and returns the index(position) of the the first element that passes a test. If the method did'nt find a match then returns -1.
Use the indexOf() method to find an element's index in an array. If the element is not found, it returns '-1'.