javascript - Why is if (undefined) evaluating to true? - Stack Overflow

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I'm new to JavaScript and have hit what seems to be an obvious issue.

const item = sessionStorage.getItem('item'); // undefined
if (item) {
    return item;
}

In the Chrome debugger I can see that item is ing back as undefined. I would expect the conditional to fail. To my surprise it's evaluating to true.

I thought I'd already tested this out. Why would it be ing back as true in the conditional?

I've e across other checks such as those remended by How to check for "undefined" in JavaScript?. But... why do I have to do anything even remotely 'plex' for a simple check like this? The above approach seems so simple and elegant I'm reluctant to lose it.

I'm also seeing this approach in an example on .

I'm new to JavaScript and have hit what seems to be an obvious issue.

const item = sessionStorage.getItem('item'); // undefined
if (item) {
    return item;
}

In the Chrome debugger I can see that item is ing back as undefined. I would expect the conditional to fail. To my surprise it's evaluating to true.

I thought I'd already tested this out. Why would it be ing back as true in the conditional?

I've e across other checks such as those remended by How to check for "undefined" in JavaScript?. But... why do I have to do anything even remotely 'plex' for a simple check like this? The above approach seems so simple and elegant I'm reluctant to lose it.

I'm also seeing this approach in an example on https://developer.mozilla/en-US/docs/Web/API/Window/sessionStorage.

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• undefined

Share Improve this question Follow edited Nov 14, 2018 at 22:31 Paul 142k2828 gold badges284284 silver badges271271 bronze badges asked Nov 14, 2018 at 22:25 Shane GannonShane Gannon 7,75699 gold badges4646 silver badges7373 bronze badges 0

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1 Answer 1

Sorted by: Reset to default 13

You're not getting the value undefined from sessionStorage, but the string "undefined", which is truthy just like any other non-empty string.

I know you're not getting the value undefined because that is impossible. sessionStorage.getItem can only return a string or null. Also note that setItem will coerce its argument to a string, so sessionStorage.setItem('item', undefined) sets item to the string "undefined".

If you want to remove an item you should use sessionStorage.removeItem('item'); instead of setting it to undefined.

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