The indexOf() method returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex. Returns -1 if the value is not found.

Syntax

str.indexOf(searchValue[, *fromIndex*]);

searchValue A character/string whose index is to be found.

fromIndex Optional. The location within the calling string to start the search from. It can be any integer. The default value is 0. If fromIndex < 0 the entire string is searched (same as passing 0). If fromIndex >= str.length, the method will return -1 unless searchValue is an empty string in which case str.length is returned.

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character of a string called str is str.length - 1.

Finding a character in a string

var str1 = "Hello";
console.log(str1.indexOf('H'));
// Console will output: 0

var str1 = "Hello";
console.log(str1.indexOf('Y'));
// Console will output: -1.

indexOf() is case-sensitive

var str1 = "Hello";
console.log(str1.indexOf('ello'));
// Console will output 1

var str1 = "Hello";
console.log(str1.indexOf('Ello'));
// Console will output -1

indexOf() with fromIndex

var str1 = "FreeCodeCamp is a place for people to learn";
console.log(str1.indexOf('Camp'));
// Console will output 8

console.log(str1.indexOf('Camp', 9));
// Console will output -1

Source MDN