To make it more clear (or more confusing, whatever you prefer

): Custom scripts are a way to write JASS directly in GUI.
The thing is that GUI is the exact same thing as JASS, but then with a lay-out and some buttons. GUI only has a limited capability though (not all functions have been converted to the buttons you're used to), so you solve that incapability by using custom scripts

.
And everyone "has" JASS. Warcraft 3 cannot read GUI, it can only read JASS.
Don't confuse people, you really can't

.
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