Javascript: what does function(_) mean

Question

I'm going through the bacon.js slide at: http://raimohanska.github.io/bacon.js-slides/1.html

In the 1st line of the 2nd block, it says:

function always(value) { return function(_) { return value } }

what does function(_) mean?

Answer 1

In this case _ is just a function parameter - a single underscore is a convention used by some programmers to indicate "ignore this binding/parameter".

Since JavaScript doesn't do parameter-count checking the parameter could have been omitted entirely. Such a "throw-away" identifier is found more commonly in other languages, but consider a case like arr.forEach(function (_, i) {..}) where _ indicates the first parameter is not to be used.

Answer 2

It's an anonymous function with one argument, the name of that argument is _.

I don't know why they bother with the argument, since the function doesn't use it.

Answer 3

It's the same as putting any other identifier to a list of arguments according to this document: http://mathiasbynens.be/notes/javascript-identifiers

You'll find in this doc that _ is a legal character that an identifier can start with.

There is no any meaning for this in your example, probably the author just thought that it's cooler than just ().