undefined | Better HN

0 pointschrismorgan5y ago0 comments

> Is there anywhere where the JS standard library uses null?

Plenty, though it depends a bit on what you count as JS standard library.

An object’s prototype may be null, so things like Object.getPrototypeOf(), Object.setPrototypeOf(), Object.create() and .__proto__ all deal with null.

Anything that is accessing a named property that exists on a class will yield null for no value, never undefined.

This is seen in DOM methods; things like Node.firstChild and Node.parentNode spring to mind as potentially null, and in each case it would be semantically quite wrong for them to be undefined—they are defined, as “no value”.

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4 comments · 2 top-level

mantap5y ago· 2 in thread

Not sure I agree that there is some logical reason why Node.firstChild should be null for an empty element whereas e.g. array[0] should be undefined for an empty array. I think the actual reason is just that DOM is an old API. If designed today it would return undefined.

chrismorganOP5y ago

> If designed today it would return undefined.

Absolutely not.

Null means it knows what you’re asking for, and there’s no value.

Undefined means it doesn’t know what you’re asking for. (This includes function calls that don’t return a value, since you’re asking for a value that wasn’t produced—again different from null, which is explicitly producing a value.)

Node.firstChild being null for a node with no children is absolutely correct. Undefined would be incorrect.

Seeking past the end of an array finds elements that have not been defined, so undefined is correct and null would be incorrect.

Master_Odin5y ago

Except in these cases, undefined still can make complete sense if you're willing to rephrase things (like sindresorhus is).

For your Node.firstChild example, one could say that if a node has no children, then it's firstChild is not defined as it doesn't exist, and so undefined makes sense. You can construct similar plausible arguments for other usages of null, though you cannot do the inverse.

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afiori5y ago

while I agree with your position the criticism of null can also be reformulated as: are there places in common usages where both null and undefined are common/reasonable values and have different meaning?

conceptually I think so, but I am not sure how much it actually helps with the language ergonomic.

in hindsight I think undefined should have been more powerful, eg setting a property to unrefined should effectively remove the property from an object (which would also affect the prototype chain)

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