[nailer]

This is a great idea but ES2017 instead of ES2015 would make it even easier for newcomers. This would allow `await` which is a lot easier for newcomers to follow than callbacks or .then(). Also you really want string.includes() !

Node and all current browsers support ES2017 so you don't really lose anything either.

[bmn__]

includes is the same as indexof, what's the big deal in your opinion?

[nailer]

Index of element is greater than minus one is an odd way of thinking about whether an array contains an element.