The `.lock` function takes two optional parameters: `alertMessage` and `customHandler`. `customHandler` is a callback invoked when a content change is attempted

The `.lock` function takes two optional parameters: `alertMessage` and `customHandler`. `customHandler` is a callback invoked when a content change is attempted. `alertMessage` is a convenience function that calls `alert`; it could be trivially implemented by the client programmer using `customHandler`. The author is ostensibly suggesting that `alert` is a common use case. I think he's trolling us. EDIT: Just saw that the author is a company. There goes my theory.