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"compilerOptions": {
"strict": true
}
Always. 
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Others have mentioned TypeScript, but to catch an error like this, you don't even need to go that far.
Set up ESLint with a good rule set (I like the one from Walmart Labs), and it'll scream at you if you forget to use var. Actually, with that rule set, it'll probably scream at you if you use var at all instead of let, and it'll scream at you again if you used let when you could have used const. It's a tad annoying at first, but it quickly drills good habits into you.
I love TypeScript too, but I find that sometimes, I just prefer to work in dynamically typed JavaScript over TypeScript - and when I do, ESLint helps me catch errors caused by typos and omissions. You'll need to use an editor that supports ESLint, but most of them do.
As for why the JS interpreter didn't catch this for you: you gave it syntactically valid code, and it just did what you asked it to!  I know that's an annoying answer, but with JavaScript approaching its 25th birthday, it has lots of historical baggage and bits of the language that are probably best avoided. Hence my love of ESLint - it's a friendly pedant that reminds when when you step out of bounds.
TypeScript is also great, of course. And you can use TSLint too, to ensure your TypeScript code is as pedantically great as possible. 
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Ryan Peden wrote: Set up ESLint with a good rule set
That's a great idea. I will definitely look into it. Great post. Thanks.
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It's not just JS, there's other languages where the compiler just chugs along trying to make sense of whatever code is there. VB isn't much better (although option strict and option explicit help). Or what about C? Ever had a nonsensical (it seems) error 14 lines later than where you made the mistake? And that's supposed to be THE system language on the planet.
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Member 9167057 wrote: It's not just JS, there's other languages where the compiler just chugs along
Yeah, it's true. It's just this re-definition thing really leaves a mark.
Plus I'm very spoiled by VStudio and C#. It's quite good actually.
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At my first employer as a very junior software engineer (though we called ourselves programmers back then - early 1990s), we used a variant of the PICK operating system called UniVERSE that ran on AIX.
Our code was mainly in PICK BASIC, which was a "business BASIC" variant.
With the version we were using, we'd sometimes run into bugs that would throw an error at runtime, but when you'd run it in the debugger (which was a nice step-wise debugger, with tons of nice features for the time), it would pass right by the line with the supposed error, and work just fine.
Then you'd exit the debugger, run the same piece of code - and it would work perfectly!
Think about that - it was a literal debugger!
Even so, it was hella annoying...
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The heisenbug, a bug disappearing when being looked at. My own experience with such bugs boils down to mostly 2 situations:
1. Timing conditions, the debugger (by principle) slows code down. Solution: Introduce a Sleep(300).
2. Code design to behave differently under a debugger. I remember inheriting a piece of code written like that, cursing loud enough to get asked WTF is going on my several colleagues, throwing this piece of trash away and reimplementing the same functionality anew.
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This is just a tooling problem. When writing Javascript, you always use a linter. (Even when writing Typescript, you can use Tslint)
Also: don't use var anymore, use const or let. And use a transpiler if you need to support old IE versions.
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Jeroen_R wrote: Also: don't use var anymore, use const or let. And use a transpiler if you need to support old IE versions.
JavaScript and new rules for a an old JavaScript prototype builder. 
Also, I don't want to support old IE.
It's dead, Jim!
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raddevus wrote: It's only 1000 lines of code. (That's like 4 printed pages. Not bad.)
Only if you have VERY good eyes, because it would be printed in ... Courier New point 2 
I'm guessing you actually mean 20 printed pages in Courier New point 9 (which is also not bad) 
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V. wrote: Only if you have VERY good eyes, because it would be printed in ... Courier New point 2
Hahaha! You are right. I was thinking 250 words per page.
I'm nuts!! 20 pages!! What's going on around here? Oh man, I got to go back and slim things down.
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raddevus wrote: new room({location:i}) just sends in a json object which is used to initialize the object.
‘{location:i}’ is Javascript syntax for defining an object.
JSON is a format for persisting Javascript objects as text.
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jarvisa wrote:
‘{location:i}’ is Javascript syntax for defining an object.
JSON is a format for persisting Javascript objects as text.
That's a very good (specific) point.
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A poor craftsman blames his tools.
Javascript (or any language) is as good or bad as the developer using it. You can paint yourself in a corner using any language the only difference is the color of the paint!
Sorry guys, but tell me that ain't the truth. 
If you think hiring a professional is expensive, wait until you hire an amateur! - Red Adair
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ClockMeister wrote: A poor craftsman blames his tools.
ClockMeister wrote: Sorry guys, but tell me that ain't the truth.
LIES!! All LIES!!
Yeah, I totally agree with you actually. However, sometimes certain tools need a spark guard or a wires that are covered with insulator that protects me a bit.
Sitting In Tub With Toaster
I mean, yes, if I'm sitting in the tub holding a plugged in toaster, then I'm the toast. But if my toaster kills me for touching it while it's toasting that's another thing.
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A good craftsman blames himself, for not bringing the right tols to the job.
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raddevus, thank you for sharing your story! This is exemplary case why JS is a time bomb, not a language!
JS made for children, meaning when child say "I want that!" (despite he allowed or not), JS just silently does "that". It's horrible practice and strictly disallowed in enterprise code. That's why we use "statically typing" languages - they give you 100% guarantee you don't call "toaster.GimmeCoffe" - simply because it cannot!
I feel a big sorry for poor JS developers - they chose so horrible language and waste so much time on learning, developing, debugging... Guys, what for?? Are you masochists?? 
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JavaScript is like virginity. It is its' own punishment.
Really and Truly.....
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I love JS. Does that make me NOT a developer?
Everyone is born right handed. Only the strongest overcome it.
Fight for left-handed rights and hand equality.
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ZurdoDev wrote: I love JS. Does that make me NOT a developer?
No, no. Of course not.
I mean, if you become addicted to any substance, does that make you not an addict?
I'll let you answer that for myself.
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ouch
Everyone is born right handed. Only the strongest overcome it.
Fight for left-handed rights and hand equality.
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ZurdoDev wrote: ouch
Please don't let it spread around, but I really like JavaScript too.
I like it because it is proliferate --- browsers are everywhere -- you can write code anywhere!
I just wish it had some more and better tools for debugging and compiler warnings so development with it could be more structured and systematic. But sometimes I just really like it because it is so wild and zany too. It's so dynamic you can do crazy cool things with it.
Also, the Canvas (drawing / gaming) stuff that you can do is amazing and quite easy to learn.
Check out my robotDots article (HTML5 Canvas : Clean JavaScript & Code Organization Allows Faster Dev, Easier Extensibility[^]) or my console in a web page article (Command Console in your browser via HTML5 Canvas & JavaScript[^]) here at CP.
Of course you are a real dev. 
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I agree. Well said.
Everyone is born right handed. Only the strongest overcome it.
Fight for left-handed rights and hand equality.
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raddevus wrote: ...in this case the JavaScript compiler...
That might be your problem, you're compiling and interpreted language!
Seriously, though, if you're concerned with size (as mentioned above) just minify the production version.
"Never attribute to malice that which can be explained by stupidity."
- Hanlon's Razor
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