since `insertFn` is optional, it should be noted as such

[ljharb]

The signature for this method should be (key, updateFn[, insertFn]) since you're able to omit it.

[bmeck]

insertFn isn't optional if you omit updateFn though. It's a bit odd. Since you need at least 1 fn insertFN is only optional in light of the value of updateFn and cannot always be omitted.

[ljharb]

right, but omitting updateFn requires an explicit undefined - so the .length of the function is 2, because 2 arguments are required at minimum. That there's validation performed on the 2nd, and that its value may force a 3rd to be required, is separate.