Uncollectable reference cycles are shockingly easy to make in JS, especially with React. A classic example:

    function closure() {
        var smallObject = 3;
        var largeObject = Array(1000000);

        function longLived() { return smallObject; }
        function shortLived() { return largeObject; }
        shortLived(); return longLived;
    }

Will keep largeObject alive.