Preserving order of fields when parsing

I understand that Object in JS has no concept of order, but Map does. Since the keys in config yaml file are listed in certain order, there are times when that order matters (i.e. if the yaml file is a set of instructions rather than config). Is there a way to tell the parser to preserve that order and store the data in Map instead of Object? I was hoping for something like a flag/option to safeLoad, but did not see one.

[puzrin]

Please, just provide one example from real world, as been requested. This repo is not for discussion about JS, and there are no plans to add maps just for fun.

[atsepkov]

It’s not so much that it’s not preserved, but that its preservation is a side-effect of current implementation on a particular platform. It’s not guaranteed to be preserved because Object has no concept of order: https://stackoverflow.com/questions/5525795/does-javascript-guarantee-object-property-order. If Object.keys/entries/values() implementation were to change tomorrow in node/Firefox/Chrome, or is implemented using a different algorithm in another JS environment, the order may come out differently. This could result from something as simple as Object.keys() deciding to sort results by key’s hash value.

It is one of the main reasons Map (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) was added into the spec, which works the same way but guarantees order. I understand that the logic within js-yaml already scans keys in the same order as they are listed in the yaml file, but once that data goes into {}, js-yaml can’t guarantee that the underlying JS engine won’t do something funky with it (i.e. WebAssembly integration/optimization).

Looking under the hood (and sorry if I’m wrong), it looks like js-yaml uses traditional Object (https://github.com/nodeca/js-yaml/blob/master/lib/js-yaml/dumper.js#L692) to represent the underlying node structure. If there was a flag to switch the underlying structure to a Map (with all other things being the same), then the user could guarantee order on any version of JS.