JS: String.prototype.lastIndexOf

s.lastIndexOf(str)

s.lastIndexOf(str, start_search_index)

Return the index where the first occurrence of first char of str, starting search at start_search_index, from right to left. start_search_index by default is 0. Returns -1 if not found.

const ss = "abcabc";

console.log ( ss.lastIndexOf ( "b" ) ); // 4

// start at index 1 (including index 1)
console.log ( ss.lastIndexOf ( "b", 1 ) ); // 1

// more than 1 char
console.log ( ss.lastIndexOf ( "bc", 2 ) ); // 1

// even if the substring pass start index, it's still considered found
console.log ( ss.lastIndexOf ( "bca", 1 ) ); // 1

// example of not found, returns -1
console.log ( ss.lastIndexOf ( "xy" ) ); // -1

Reference

ECMAScript 2015 §Text Processing#sec-string.prototype.lastindexof