Encoding and decoding a unique ID between two systems in nodejs - node.js

Question

I am writing a script which synchronizes users between two systems. Lets say the source and target system.
To help with synchronising them I was hoping to store the user ID from the source system as an ID in the target system.
Unfortunately the target system has a max character length for the property I can store this in.
If possible, i would like to avoid creating a new table to persist the relationship.
I can't truncate it as I need to be able to refer back to the user in the source system from the target system.
Is there a way of encoding and decoding the source User ID?
Edit:
The ID in the source system will always follow the structure of:
/^[a-zA-Z0-9]{6}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{3}-[a-zA-Z0-9]{6}$/
For example: as0092-banc-mdn-da1023
There are no restrictions on what can be stored in the target system apart from it having a max character length of 20.

Answer 1

When you originally posted your question, it seemed like you needed a massive conversion, ie squishing a 20 character long ID into something much shorter, that would have been quite a problem. But now after your edit it seems quite easy:
You have 20 characters available in the target system, but the ID in the source system only has 19 variable characters plus 3 dashes on fixed positions.
So when converting from source to target just remove the dashes. This will give you a string of length 19, which will perfectly fit into your 20 character wide field on the target system. And as the dashes are at fixed positions, they don't carry any semantics, ie removing them won't tamper with uniqueness of your ID.
And when converting from target back to source, you know where in your ID the dashes have to go, ie after the 6th, the 10th and the 13th character of your squished ID. So just put them back in there at the right places, and you have your ID format of the source system back ...
let
original = 'as0092-banc-mdn-da1023', //this is the key from your example
target = original.replace(/-/g, ""), //this removes all dashes
source = target.slice(0, 6) + "-" //this takes certain portions of your squished ID
+ target.slice(6, 10) + "-" //and inserts dashes between them
+ target.slice(10, 13) + "-"
+ target.slice(13)
console.log("original ", original, " length", original.length);
//the mapped ID fits perfectly into your target system's length restriction
console.log("target ", target, " length", target.length);
//the reversed mapping is equal to your original ID
console.log("source ", source, " length", source.length);
console.log("test eq ", source === original);
Of course there are other possibilities, to remove and readd the dashes, but I think you get the idea behind the code.