Template to Func

Written by DAZ on Wed, 4 Jun 2025

Yesterday at work, somebody posted this JavaScript snippet, saying they liked how the some method worked:

const anyChecked = filterTargets.some(filter => filter.checked)

Someone replied, saying they preferred how to do it in Ruby:

any_checked = filter_targets.any?(&:checked)

I had already implemented the any in my Ruby Doo library, but I also like the way you can use the symbol to proc shorthand notation in Ruby to automatically call a method on each element passed to it. Interesting fact that this was originally a Rails method that was then merged into Ruby core as it proved so useful. It does seems a lot of needless code writing filter => filter.checked when all you want to do is apply a method to each element in the array.

JavaScript already allows you to use a point free style where you can just provide the name of a function and each element in the array will be passed to it, so we could do this:

const filterChecker = filter => filter.checked
const anyChecked = filterTargets.some(filterChecker)

But in this case, we have just shifted the problem to defining a trivial function, filterChecker, that simply calls a method on the object provided to it. The problem arises if you are doing a lot of OOP style coding, then you will often be calling methods on objects rather than top-level functions. I wondered if it was possible to implement something similar to Ruby's symbol to proc in Ruby Doo.

My first effort looked like this:

const sym_to_proc = method => obj => obj[method]

This is a curried function that uses partial application to return another function that can then be passed as a callback to the array method. This means you coudl write this:

const anyChecked = filterTargets.some(sym_to_proc("checked"))

This looks a bit messy though ... the Ruby version uses the ampersand to start the call and it's just followed by a symbol. Having to use the extra parentheses to call the function is also messy. So I got creative ... JavaScript doesn't have symbols, but it does have template literals, and they don't require parentheses when you call a function with them as the argument (tag functions). Unfortunately you can't use an ampersand for a function name, but as jQuery showed, you can use $, so I rewrote the function as this:

const $ = method => obj => obj[method]

Now we can call it like this:

const anyChecked = filterTargets.some($ `checked`)

... which, while not exactly the same, is much closer to the Ruby version.

So I added it to an updated version of Ruby Doo, so now you can write this:

const anyChecked = filter_targets.any($ `checked`)

I'm not sure how much it would get used, but it was fun to implement and is more useful when using Ruby Doo, which adds a lot more methods to JavaScript objects. I like the fact it uses tag template functions in a creative way and the use of the $ is similar to the way it's used in template literal interpolation. And I decided to call it Template to Func, in honour of the Ruby method that inspired it, since it uses template literals instead of symbols and functinos instead of procs.