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    <script>
        Number.prototype._isPrime = function (num) {
            let numSqrt = parseInt(Math.sqrt(num));
            for (let i = 2; i <= numSqrt; i++) {
                if (num % i == 0) return false;
            }
            return true;
        }

        var res = Number.prototype._isPrime(9);
        console.log(res)
    </script>
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