# Cons of Callback
# 1. callback hell
doA('url', function(){ doB('url', function(){ doC('url', function(){ doD('url', function(){ ... }) }) }) })Copied!
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# 2. hard to understand
people used to think were alike block model, which contrast with async model. it could make developers feel complex.
# 3. inversion of control
A part of your code will be under third party's control.