Switch function's scope, return new function

switch-scope.js

function switchScope(fn, params) {
    var keys, values;

    keys = Object.keys(params);

    values = keys.map(function (val) {
        return params[val];
    });


    keys.push("return " + fn);

    return Function
        .apply(null, keys)
        .apply(null, values);
}

var a = 1, b = 2,
    add = function () { return a + b },
    res1, res2;

res1 = add();
add = switchScope(add, {a: 3, b: 4});
res2 = add();

console.log(res1, res2);