Gives the position of the first occurrence of the string passed as parameter in the current string. Returns -1 if the string is not found.

'JavaScript'.indexOf('Script') //4
'JavaScript'.indexOf('JavaScript') //0
'JavaScript'.indexOf('aSc') //3
'JavaScript'.indexOf('C++') //-1

You can pass a second parameters to set the starting point:

'a nice string'.indexOf('nice') !== -1 //true
'a nice string'.indexOf('nice', 3) !== -1 //false
'a nice string'.indexOf('nice', 2) !== -1 //true