wrapper

snippets.js

/**
 * Wraps a `Function` with another `Function`. This allows
 * the wrapper function to intercept and manipulate the
 * return value of the orginal function.
 * @param  {Function} fn      The function to wrap
 * @param  {Function} wrapper The wrapper function
 * @return {Function}         Return the wrapped function
 */
function wrap(fn, wrapper) {
  return function() {
    var args = slice.apply(arguments);
    return wrapper.apply(this, [fn].concat(args));
  };
}

// Usage
var wrappedFn = wrap(sum, function() {
    var args = sp.slice.apply(arguments);
    // First argument is the wrapped function
    var originalFn = args.shift();
    return originalFn.apply(this, args) * 2;
});

expect(wrappedFn(1, 2, 3, 4)).toEqual(20);