adding try catch fail

Author: MayaGans

figures/try-catch-fail.pdf

@@ -326,7 +326,22 @@ \section{Promises}\label{s:promises-promises}
 
 \noindent
 then the error message won't appear
-because the function \texttt{fetch} is asynchronous. 
+because the function \texttt{fetch} is asynchronous,
+which means that code executes in this order:
+
+\begin{enumerate}
+\item
+  inside try, ask the computer to execute \texttt{fetch(url)} at some point in the future
+\item
+  since asking this doesn't cause an exception, skip the catch
+\item
+  then run \texttt{fetch(url)},
+  which fails without anything in place to catch the exception 
+  because the \texttt{try...catch} has already finished 
+  so \texttt{console.log} never runs and the error message doesn't appear.
+\end{enumerate}
+
+\figpdf{figures/try-catch-fail.pdf}{When Try...Catch Fails}{f:try-catch-fail}
 
 There are two passes of execution in this block:
 The first is the \texttt{try...catch} event loop,