compile() should throw an error if it reaches a helper fn that is not registered

[CnovakChromeRiver]

I was debugging some templates another dev wrote which were not working. He was prototyping with Handelbars for proof of concept and to see how it worked. His custom block helper was not working. We read through the docs and looked for JS errors in the console (there were none) and attempted to debug what was going on.

After a while, I noticed he had placed his registerHelper() call after his compile() call to the source template. I suggested moving registerHelper() above the compile() assuming that the helper was not available when compile() fired. It instantly fixed everything.

The problem is, Handlebars didn't throw an error like "helper function 'list' is not registered." Instead, Handlebars silently handled the error, which was not helpful for debug purposes. Please throw errors when Handlebars encounters one, thanks!

[watsoncj]

I ran into this while pairing today. We tried to use the list helper from the documentation without realizing that it is not built in. If compile threw an error it would have saved us a ton of time trying to track down the issue.

Is this worth submitting a pull request for? Is there any reason not to throw an error?

[kpdecker]

The problem is that we can not ensure that a particularly helper is available at compile time as the helper exec is effectively late bound (i.e. you can pass in whatever helps you wish at exec time with the options.helpers field).

The knownHelpersOnly flag does basically this (it was intended for precompilation but could be useful and actually improve performance under normal compile mode) but this can only be done for cases that are unambiguously helpers, i.e. anything with a block contents or arguments. This will not help for causes like {{mightBeAHelper}} was there is no way to know what this is until we we bound with both a helper set and data set.

[psayre23]

Looks like the knownHelpersOnly flag doesn't do anything in rc3. I think line 1114 needs to be this.options.knownHelpersOnly instead of this.knownHelpersOnly

[kpdecker]

Good catch. Thanks!