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

constraints.js

// copied from https://gist.github.com/lauripiispanen/1fd1c3319084f9913f2d
// improved formatting
const constrain = ({
    pre = () => {},
    post = () => {}
}) => fn => (...args) => {
    const throwIf = x => y => {
        if (x(y)) {
            throw x(y);
        } else {
            return y;
        }
    };
    const compose = (...args) => args.reduce((x,f n) => (...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))