Clojure-style :pre and :post constraints in ES6 - big thanks to @polytypic for the original idea!

constraints.js

const constrain = ({ pre = () => {}, post = () => {} }) => fn => (...args) => {
  const throwIf = x => y => { if (x(y)) { throw x(y) } else return y },
        compose = (...args) => args.reduce((x,fn) => (...args) => fn(x.apply(x, args)))
  return compose(throwIf(pre), fn.bind(fn, args), throwIf((...a) => post.apply(a.concat(args)))).apply(null, args)
}

const sqrt = constrain(
  {pre: x => x < 0 && "Let's go shopping!",
   post: (y, x) => y < 0         && "Math is hard."
                || 0.1 < y*y - x && "Floats are harder.",
   doc: "Round root"})(Math.sqrt)

console.log(sqrt(9))