Are two strings a rotation of one another

Rotation.js

var isRotation = function(s1, s2) {
    return (s1.length === s2.length) && ((s1 + s1).indexOf(s2) !== -1);  
}; 

s1 = "stackoverflow"
s2 = "tackoverflows"
s3 = "ackoverflowst"
s4 = "overflowstack"
s5 = "ovegflgwsgacg"

console.log(isRotation(s1, s1));
// -> true
console.log(isRotation(s1, s2)); 
// -> true
console.log(isRotation(s1, s3)); 
// -> true
console.log(isRotation(s1, s4)); 
// -> true
console.log(isRotation(s1, s5)); 
// -> false