str.lastIndexOf(searchString, null) returns -1

lastIndexOfFromNull.js

// Node v13.12.0

const str = "abcde";
const searchString = "c";

const indices = [
  str.indexOf(searchString), // 2
  str.indexOf(searchString, null), // 2
  str.lastIndexOf(searchString), // 2
  str.lastIndexOf(searchString, null) // -1
];

indices.forEach((n, i) => console.log(`case${i}: ${n}`));