Kenneth Lum

Posted on Mar 21, 2020 • Edited on Mar 27, 2020

There are 9 falsy values in JavaScript inside of a browser. Can you name them all?

#javascript

Sometimes when programming in JavaScript, it is easy to get an uncertain feeling of, what in JavaScript are falsy?

It is slightly different if it is pure JavaScript versus in a browser.

When it was ES5, there were 8 falsy values. Can you name them all?

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The answer is, there were 5 primitive types in JavaScript ES5. Each primitive type contributes 1 falsy value, except Number, which contributes 3:

The way I remember them is:

NNBUS

Null, Number, Boolean, Undefined, String

And the corresponding falsy values are:

null
0
-0
NaN
false
undefined
''

So this is "falsy 7". There is one more, at the end of this post.

0 can be viewed as the same as -0 sometimes, depending on how you look at it. If you have n dollars, and n is 0 or -0, it really doesn't matter that much. But let's say, if you calculate something, and it is 1 / 0 or 1 / -0, then they give very different results:

1 / 0    // Infinity
1 / -0   // -Infinity

The operator === would compare 0 and -0 to be the same, but Object.is() would compare them as different:

0 === -0           // true
Object.is(0, -0)   // false

The other different behavior of Object.is() is that it returns true for Object.is(NaN, NaN), while NaN === NaN returns false. So in a way, you can think of Object.is() as the twin brother of === that is more precise.

You can also think of 0 not being equal to -0: when it is +0, a tiny bit of matter, it can cost you nothing, such as a grain of sand. But if it is -0, which is a tiny bit of antimatter, it would cost you a lot of money, as the current price of 1 gram of antimatter is $62.5 trillion dollars.

In ES6, there are two more new primitive types: Symbol and BigInt. Symbol is the only primitive type that doesn't have a falsy value, and BigInt also contributes one falsy value: 0n.

Now, almost all objects in JavaScript are truthy values, except one. Do you know what it is? Together, they make up the 9 values in JavaScript that are falsy, and the object is document.all, and is falsy coming from the usage when the code checks if (document.all) and document.all is falsy to tell the program: don't use document.all.

document.all     // HTMLAllCollection(3) [html, head, body]

!!document.all   // false

So the falsy values in JavaScript in a browser, 9 of them, are:

null
0
-0
NaN
false
undefined
''
0n
document.all

Top comments (10)

Zane Milakovic • Mar 21 '20

Is document.all really part of JS? I don’t think you can access it in node for example. So I would think it’s exposed via the browser api, and behavior on the browser itself.

But really great write up, thank you!

Kenneth Lum • Mar 21 '20 • Edited

I guess just a familiar JS environment (some of us grew up with)... I think the amazing thing is, if the browser complies to JS, there is no way that it can make document.all to be falsy? If there are some weird things happening in NodeJS regarding JS I'd hope to know too.

By the way... I updated the post to mention it is inside of a browser, thank you.

Wassim Chegham • Mar 22 '20

Good write-up!

One note though... Actually, technically speaking, and according to the JavaScript specs (ECMA 262), there are only 7 values that are Falsy, meaning that they are considered false when encountered in a Boolean context.
And when it comes to document.all it's not part of the JavaScript specs per se. It's part of the DOM API (W3C), which browser-based only (Node.js doesn't have a document object, nor a DOM).

Cheers ;)

Kenneth Lum • Mar 23 '20

It is 7 but not 8? Which one is not listed... (I tried to search for "falsy" or "falsey" in that docs but can't find any match)... true, document.all really is inside of a browser only, so I think I will update the post

Wassim Chegham • Mar 23 '20

You can find more info here: developer.mozilla.org/en-US/docs/G...

Kenneth Lum • Mar 23 '20

oh... not counting -0... depending on how you look at it. It can be incorrect, because 0 and -0 are technically different.

Cristian Patricio Contreras • Mar 23 '20

What about [], "\t" and "\r", I think those values are falsey too!

Kenneth Lum • Mar 23 '20 • Edited

In Google Chrome or in NodeJS:

> !![]
true

> !!"\r"
true

> !!"\t"
true

> !!"\n"
true

> Boolean([])
true

!![] is the same as Boolean([])

In AngularJS some time ago, I remember AngularJS would consider [] to be falsy but in a later version to be truthy: stackoverflow.com/questions/357763...