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

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?

Scroll down to read what they are...
　
　
　
　
　
　
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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