grunt-contrib-uglify is borking my JavaScript. What am I doing wrong?

gistfile1.js

/**
 * Before grunt-contrib-uglify
 */
(function(doc) {
    if (document.querySelector) {
        var toggleClassName = function(element, toggleClass) {
            var reg = new RegExp('(\\s|^)' + toggleClass + '(\\s|$)');
            if (!element.className.match(reg)) {
                element.className += ' ' + toggleClass;
            } else {
                element.className = element.className.replace(reg, '');
            }
        };

        doc.querySelector('a[href="#navigation"]').addEventListener('click', function(e) {
            e.preventDefault();
            toggleClassName(doc.body, 'js-menu-active');
        });
    }
}(this.document));

/**
 * After grunt-contrib-uglify (mangle:false, beautify:true)
 */
!function(doc) {
    if (document.querySelector) {
        var toggleClassName = function(element, toggleClass) {
            var reg = new RegExp("(\\s|^)" + toggleClass + "(\\s|$)");
            element.className.match(reg) ? element.className = element.className.replace(reg, "") : element.className += " " + toggleClass;
        };
        doc.querySelector('a[href="#navigation"]').addEventListener("click", function(e) {
            e.preventDefault(), toggleClassName(doc.body, "js-menu-active");
        });
    }
}(this.document);

Still debugging, but just to explain this line for you:

element.className.match(reg) ? element.className = element.className.replace(reg, "") : element.className += " " + toggleClass;

Firstly, let's break that down onto lines:

element.className.match(reg) 
  ? element.className = element.className.replace(reg, "")
  : element.className += " " + toggleClass;

This is a ternary operator. It's basically a shorthand for a conditional. These two are equivalent:

if(a) {
  doSomething();
} else {
  doOtherThing();
}

a ? doSomething() : doOtherThing();

Those two are the same. Uglify is rewriting the conditional just to save room (and hence bytes), I imagine. The only other thing it's doing is reversing the conditional. You have:

if(!foo) { something() } else { someOtherThing(); }

And it's doing this:

if(foo) { someOtherThing(); } else { something() }
// and then:
foo ? someOtherThing() : something()

HTH.