Memoized Y-Combinator in ES6

y-memo.js

let Ym = (f, s = JSON.stringify) => (g => g(g))(x => f((...v) => (k => k in x ? x[k] : x[k] = x(x)(...v))(s(v))))

let fib = Ym(f => n => n <= 1 ? n : f(n - 1) + f(n - 2))

console.log('fib(1000)', fib(1000)) // => 4.346655768693743e+208

Yeah, that's what I had found as well. I like putting JSON.stringify in as a default param, so we can use the abbreviated arrow function syntax. Thanks for your feedback!

Updated the gist. I also removed the cache param. The cache will be stored in the anonymous function x instead.

Very clever, using the recursion function as the cache.

I'm using key in cache (now k in x) so you can store false-y values in your cache without having to calculate them over and over.

It's also possible to get rid of some extraneous parentheses, leading to:

let Ym = (f, s = JSON.stringify) => (g => g(g))(x => f((...v) => (k => k in x ? x[k] : x[k] = x(x)(...v))(s(v))));

Oh, I see. I wasn't sure why you used in instead of just trying to read the cached value. Good call, I'll update the code. By the way, post your Twitter handle, so I can credit you for your contribution :)