When first developing Ghost, I ran into the helperMissing helper. For our purposes, throwing an exception was a bit much, so I added an override to make it log the error and continue.

However I've since noticed that helperMissing only gets called if you pass something to the helper:

{{test}} won't cause helperMissing to be called
{{test "testing"}} will cause helperMissing to be called
{{#test}}{{/test}} will cause blockHelperMissing to be called

This means that in some cases, using a helper that doesn't exist throws an exception, and in others, it fails silently. I realise that in the 'fails silently' case the helper could in fact be a key or path.

Having researched this a little bit, I found an issue which indicates that missing keys should cause helperMissing to be called: #318. More recently, someone raised this issue (#586) and the answer seems to me to have overlooked the fact that #318 states this should work.

Furthermore it indicates that the solution should be to override escapeExpression - however escapeExpression doesn't have any context such as the name of the helper/key which was attempted to be called, so it isn't possible to output a useful error at this point.

Context:
My ultimate goal here is to capture all of the problems (be they exceptions or silent failures) in a handlebars template, throw exceptions in debug mode, and log warnings otherwise. This is to make developing Ghost themes a little bit nicer.

I believe it ought to be possible (even if it isn't the default) to capture all issues in a template in a consistent way, and deal with them as we please. Given that there is already helperMissing and blockHelperMissing perhaps there could be a keyMissing method that gets called if the key does not exist. This method could just be a pass-through by default, but at least then users of handlebars.js could choose to override that functionality if needed.