Hi if de changes to du,de la des ... Why is it "J'ai envie de chocolat"

Question

Accepted Answer

Bonjour Alison !In the case of fixed expressions such as "avoir envie de", you're literally saying "I have envy of [something]".Using du, de la, de l' or des would add the definite article (de + le, la, l', les) and would be used to be more specific = I have envy of the [thing] (that I mentionned previously).
J'ai envie de chocolat.I feel like chocolate.J'ai envie du chocolat que tu m'avais offert pour mon anniversaire.I feel like the chocolate you'd given me for my birthday. 
I hope that's helpful!À bientôt !

Answer

Bonsoir Alison,
I think the answer lies in the fact that «avoir envie de» is a fixed expression.  In reviewing the lesson, all of the phrases construct similarly regardless of what is desired be it a masculine or feminine noun.
Bonne chance.