+= creates a new string every time.

If you have a look in the Details of the kata, it says: "The Morse code table is preloaded for you as a dictionary, feel free to use it:"

So I wondered why this person remade the dictionary.

where is the dictionary is?

Loops can be slower than comprehensions. Also, you're creating a new string on each iteration, which is bad for performance as well.

Same here ;) but the 'basic' nested loop feels easier to read isn't it ?

but, but, but, the dictionary was provided?!