brain-ex-sum100.js

    var r = '123456789'.split("").reverse().reduce(function(pre, current){
        pre = (pre.constructor == Array) ? pre : [pre];
        return ["", "+", "-"].map(function(op) {
            return pre.reduce(function(carry, item) {
                return carry.concat(
                    ("-" + current + op + item).replace(/[-,+][-,+]/g, "-"),
                    (current + op + item).replace(/[-,+][-,+]/g, "-")
                );
            },[]);
        }).reduce(function(p, c) {
            return p.concat(c);
        }, []).filter(function(item, pos, self) {
            return self.indexOf(item) == pos;
        });
    });

    console.log(r.length);
    console.log(r.filter(function(val){
        return (eval(val) === 100);
    }));