Either fail gracefully or emit useful error message .on("unknownEvent")

[syntagmatic]

Using .on to listen to an event that doesn't exist triggers an error on

d3-dispatch/src/dispatch.js

Line 19 in b2944af

if (t && !types.hasOwnProperty(t)) throw new Error;

Possible solutions:

• parseTypenames filters out undefined event types
• error alerts the full event name that couldn't be matched to an event

[mbostock]

I’m somewhat averse to error messages because the error should be pretty obvious from the stack trace, and debuggers are far more effective at debugging than reading error messages. But, okay.