1. Where did anyone say we were hitting undefined behavior? We're hitting a well-defined JavaScript value whose name is "undefined".
2. Why is crashing correct? Don't answer in terms of language specs, answer in terms of desired behavior for whatever you're trying to do with the program. Software engineering is a tool for accomplishing other things. Sometimes, yes, software crashing is the right thing in service of that other goal. But not always.
Undefined behavior of a program is when a programmer didn't define how to handle a particular situation. If an array becomes "undefined" and the programmer didn't expect such result (e.g. there's no if (typeof array === "undefined") { ... }), then the behavior is undefined regardless of what kind of types there are in JavaScript.
Crashing in such case is used to avoid incorrect functioning of the program, such as overwriting user data or introducing security vulnerabilities. By definition, if the behavior wasn't expected, the program is in unknown state. Sure, you can gracefully handle such situations (e.g. allow website to load other scripts if the particular piece that "crashes" not important for its functioning, crash just some kind-of sub-process and restart it, or — if it's user input — end up with some predefined value regardless of the incorrect input, etc.), but the correct _default_ behavior is to crash, otherwise you'll end up with unknown state and the algorithm that you wrote will be incorrect.
I recommend anyone who wants to see how much unexpected behavior is in their JavaScript programs to install typescript@next and start adding types, compiling with tsc --strictNullChecks.
1. Where did anyone say we were hitting undefined behavior? We're hitting a well-defined JavaScript value whose name is "undefined".
2. Why is crashing correct? Don't answer in terms of language specs, answer in terms of desired behavior for whatever you're trying to do with the program. Software engineering is a tool for accomplishing other things. Sometimes, yes, software crashing is the right thing in service of that other goal. But not always.