[Bug?] undefined is converted to null

[marvin-j97]

Version

Using 1.10.0

Reproduce

const YAML = require("yaml");

const obj = () => ({
  a: 4,
  b: "str",
  c: null,
  d: undefined,
});

const jsonified = JSON.parse(JSON.stringify(obj()));
console.log(jsonified.hasOwnProperty("d")); // -> false
console.log(jsonified["d"]); // -> undefined

const yamlified = YAML.parse(YAML.stringify(obj()));
console.log(yamlified.hasOwnProperty("d")); // -> true???
console.log(yamlified["d"]); // -> null

I don't see how this would be intentional. undefined is not equal to null.

[eemeli]

This was a design choice of mapping undefined to null in YAML, as it's the closest match to an empty value. But I do see your point that this is different from what JSON.stringify() does.

I think this should be considered in terms of the 2.0 update, as changing the API like this can be a breaking change.

[kyeotic]

I would like to vote that undefined keys not remain in the stringified version. I am now manually preprocessing to remove these keys, deeply.

[eemeli]

Yeah, happy to include this fix in the major update. A PR on this would be welcome; I'm finding myself spending a bit more time on a couple of other projects atm.

[kyeotic]

@eemeli I tried to dig into this. The code is pretty complicated, but it looks like some information is lost when construction the node graph so that undefined is converted into null as the Pair is made. That means the change can't be isolated to stringify, it needs to be part of the document's construction.

[eemeli]

@kyeotic The right place to fix this was in tags/failsafe/map.js, where the JS Object's entries are added to the new mapping. By the time we're creating the Pair it's already too late.

JSON.stringify([1, undefined, 2]) === '[1,null,2]', so it makes sense to also have this behaviour be object-specific in yaml.

I also opted to keep undefined values in Map objects as such, because I figured that's an intentional choice from the user.