Cast "null" as null in javascript

Question

I use the session storage to set a key that can take numeric values or null. I noticed that the field is stored as a string no matter what I put in. Is there a nice way to convert back my stored value to null ?

For instance, I want to do a check in my code as such:

if (!sessionStorage.getItem('key')) {
   // do something
}

Whats the best way to make that check? The obvious way would be

 if (!sessionStorage.getItem('key') !== "null") {
       // do something
 }

I am wondering if there's an alternative.

Answer 1

You can parse it

JSON.parse('null') // null
JSON.parse('1')   //  1

so

var val = sessionStorage.getItem('key');
val = JSON.parse(val);

Close to cast

Alternative would be:

var val = sessionStorage.getItem('key');
val = val*1 || null;

Answer 2

I just want something that would evaluate to false

You could use an empty string instead to mark the unset value:

sessionStorage.setItem('key','');

This way you can re-use your current check:

if (!sessionStorage.getItem('key')) {
   // do something
}

---

That said, you mentioned you had a drop down to select values, and that's why you need such a value, but I think a solution involving deleting the item on the null value would be better. Here's a working demo, and the code used to make it:

var dd = document.getElementById('dropdown');

dd.addEventListener('change', function(){
  var value = this.value;
  if (value === 'null')
    sessionStorage.removeItem('key');
  else sessionStorage.setItem('key', value);
});

var check = document.getElementById('check');
check.addEventListener('click', function(){
    this.innerHTML = 'Value: '+sessionStorage.getItem('key')+
                     ', check: '+(!sessionStorage.getItem('key'));
});

<select id="dropdown">
  <option value="null">none</option>
  <option>1</option>
  <option>2</option>
  <option>3</option>
  <option>4</option>
  <option>5</option>
</select>

<button id="check">Check value</button>