CreateReactiveNumber.js

var CreateReactiveNumber = function(initValue, min, max, callback) {
    var _value = initValue;

    var value = function(newVal) {
        if (arguments.length > 0) {
            var oldVal = _value;
            _value = newVal;
            if ((oldVal > min && oldVal < max) !== (newVal > min && newVal < max)) {
                callback((newVal > min && newVal < max) ? 'enter' : 'leave', _value);
            }
            return _value;
        } else {
            return _value;
        }
    }

    return value;
};

test.js

var something = CreateReactiveNumber(0, 10, 20, function(action, value) {
    console.log('number "' + action + '" range(10, 20)');
});

// change value in your game
console.log('start value is ' + something());
something(12); // 'enter'
something(18); // no log
something(24); // 'leave'