JS: String.prototype.indexOf

s.indexOf(str)

s.indexOf(str, start_search_index)

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

const ss = "abcabc";

// return index where first b occurs
console.log ( ss.indexOf ( "b" ) ); // 1

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

// start at index 2
console.log ( ss.indexOf ( "b", 2 ) ); // 4

// more than 1 char
console.log ( ss.indexOf ( "bc", 2 ) ); // 4

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

Reference

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