Mongo : Custom system.js in find(), like query

Question

trying to write a Mongo query that will Base64 Decode a field that is Base64 encoded and then perform a simple "like" on the decoded value. I'm following a couple of different posts as well as the Mongo docs, but can't seem to get the syntax correct. I basically want to do a query like this :

db.getCollection('my-collection').find (
     { base64Decode(edmDocumentId): /ni-billing-retro/ }
);

Where base64Decode() is a custom function inserted into system.js.

Posts:
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Export text stored as Bindata in mongodb
How to query MongoDB with "like"?

What I've done so far :

• I saved the base64Decode() function to the system.js, and I can see the function....https://docs.mongodb.com/manual/tutorial/store-javascript-function-on-server/.

    db.system.js.insertOne( {
        _id: "base64Decode",
        value : function (s) {
            var e={},i,k,v=[],r='',w=String.fromCharCode,u=0;
            var n=[[65,91],[97,123],[48,58],[43,44],[47,48]];

            for(z in n){for(i=n[z][0];i<n[z][1];i++){v.push(w(i));}}
            for(i=0;i<64;i++){e[v[i]]=i;}
            function a(c){
                if(c<128)r+=w(c);else if(c>=192)u=c;else r+=w(((u&31)<<6)+(c&63));
            }
    
            for(i=0;i<s.length;i+=72){
                var b=0,c,x,l=0,o=s.substring(i,i+72);
                for(x=0;x<o.length;x++){
                    c=e[o.charAt(x)];b=(b<<6)+c;l+=6;
                    while(l>=8)a((b>>>(l-=8))%256);
                }
            }
            return r;
        }
    });

• I've tried using $where, to no avail...returns ReferenceError: edmDocumentId is not. Added the db.loadServerScripts(); to fix the base64Decode() Reference error.

    db.loadServerScripts();
    db.getCollection('rapid-document-meta').find (
         { $where: (base64Decode(edmDocumentId) == /ni-billing/) }
    );

• I've tried doing a straight find (), Unexpected token : Line 2

    db.getCollection('rapid-document-meta').find (
        { base64Decode(edmDocumentId): /ni-billing-retro/ }
    );

• Tried the following from Calling db.system.js Function in $where : ReferenceError: edDocumentId is not defined, even though edmDocumentId is on every single record.

db.loadServerScripts();
db.getCollection('rapid-document-meta').mapReduce (
    base64Decode(edDocumentId), 
    function() {},
    { "out": { "inline": 1 } }
);

Does someone have an example of a find query that uses a custom function from system.js??? Mongo version 4.0.8.