pattern-match.js

const otherwise = true;
const isNumber = n => typeof n === 'number';
const eq = a => b => a === b;


const fib = match([
    eq(0), 1,
    eq(1), 1,
    x < 0, 0,
    isNumber, () => fib(x-1) + fib(x-2),
    otherwise, () => {throw new Error('Not a number');}
]);

function match(conds) {
    return (x) => {
        if (conds.length % 2) throw new Error('array must have an even length.');
        for (let i = 0; i < conds.length; i+=2) {
            const cond = conds[i];
            let pass = eval(cond, x);        
            if (!pass) continue;
            return eval(conds[i+1], pass);
        }
        throw new Error(`Unmatched value: ${x}`);
    }
    function eval(expr, arg) {
        if (typeof expr === 'function') {
            return expr(arg)
        }
        return expr;
    }
}