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function Interval(fn, time) {
    var timer = false;
    this.start = function () {
        if (!this.isRunning())
            timer = setInterval(fn, time);
    };
    this.stop = function () {
        clearInterval(timer);
        timer = false;
    };
    this.isRunning = function () {
        return timer !== false;
    };
}

var i = new Interval(fncName, 1000);
i.start();

if (i.isRunning())
    // ...

i.stop();