Number.prototype.isBetween implementation...

gistfile1.js

/**
 * Number#between(low, high[, inclusive = false]) -> boolean
 * - low(Number): Number must be greater than `low` value
 * - high(Number): Number must be less than `high` value
 * - inclusive(Boolean): [Optional] Include low and high numbers in the check.
 *                       Defaults to `false`.
 *
 * Checks if the Number is between `low` and `high` values. Returns `true` or `false`
 **/
Number.prototype.isBetween = function(low, high, inclusive) {
	if (inclusive) {
		return this >= low && this <= high;
	}
	return this > low && this < high;
}

var five = 5;
console.log(five.isBetween(3, 10)); // true
console.log(five.isBetween(9, 10)); // false
console.log(five.isBetween(5, 10)); // false
console.log(five.isBetween(5, 10, true)); // true
console.log(Number.MIN_VALUE.isBetween(0, 0.1)); // true