how do i get the index of the element returned with match javascripte code example

Example 1: javasript array indexof

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

Example 2: js index to index

let array = [5, "Foo", 3, "Bar"];
array.slice(2);
// Output = [3, "Bar"]

let string = "Volvo Cars";
string.slice(2);
// Output = "lvo cars"