Untitled

 <body>

    <div>
            <input name="other12" id="input_extrafl12" class="input_extraf2" value="" type="number" />Довжина паролю<br>
            <input name="other13" id="input_extrafl13" class="input_extraf3" value="" type="text"/>
            <label><input id="ispen" name="is_pending" type="checkbox">Наявність цифр</label>
   </div>
    <div>
            <input name="other14" id="input_extrafl14" class="input_extraf4" value="" type="text">
            <label><input id="isopen" name="is_openinging" type="checkbox"> Наявність символів</label>

    </div>
    <button class ='gen' >Generate password!</button>;
    <hr>
    <div class="result"></div>
    <script>
    	const $result = document.querySelector('.result');
    	const $passwordButton = document.querySelector('.gen');
		const $a= document.getElementById('input_extrafl12');
		const $b= document.getElementById('ispen');
		const $c= document.getElementById('isopen');


    	$passwordButton.addEventListener('click', () => {
			const count = $a.value;
   			$result.innerHTML = count;
			if($b.checked)
				//...
			if($c.checked)
				//...
		});
    </script>
</body>
</html>