Are JavaScript strings immutable?

The string str1 is copied to another variable str2, then changed to a different string, and then it's first element str1[0] is assigned a different value:

var str1 = 'owls';

// Copy str1 to str2
var str2 = str1;

// Change str1
str1 = 'hello';

// Change str1[0]. ( Requires ES6 )
str1[0] = 'H'; 

What will be printed when we log str1 and str2?

console.log(str1 + ' ' + str2);