Use the @options.smooth directly if the value's type is "boolean".

[usualoma]

@options.smooth is true or @options.ykeys[i] in @options.smooth

This code is compiled to

var __indexOf = [].indexOf;
typeof this.options.smooth === "boolean" ? this.options.smooth : (_ref1 = this.options.ykeys[i], __indexOf.call(this.options.smooth, _ref1) >= 0);

In modern browsers or in v8, [].indexOf.call(false, value) returns -1.
But, if Array.prototype.indexOf is implemented (often in IE8) by user, that code sometimes makes error.

I think that should check by typeof @options.smooth is "boolean", instead of is true.

[oesmith]

Great work, thanks!