Not an issue, you need to read the descriptions more carefully ;-)

Thanks, I misunderstood the directions.

Well, take isMerge('codewars', 'cwdr', 'oeas'). If you try to arrange the three strings as in the description, you'll see that it doesn't work for 'cwdr':

s      = c o d e w a r s
part1  = c   d   w    r    <-- becomes "cdwr" instead of "cwdr"
part2  =   o   e   a   s

Order is important. And sorting everything discards order.

It is not clear when zeroes should be left;

Test.assert_equals(increment_string("foobar99"), "foobar100")
Test.assert_equals(increment_string("foobar099"), "foobar100")

javascript
The test cases are still mixed on whether or not the function is supposed to preserve leading zeros:
Test Results:
Log
foobar000
Test Passed: Value == 'foobar001'
Log
foo
Test Passed: Value == 'foo1'
Log
foobar001
Test Passed: Value == 'foobar002'
Log
foobar99
Test Passed: Value == 'foobar100'
Log
foobar099
Expected: 'foobar100', instead got: 'foobar0100'
Test Passed: Value == '1'