Authenticating with MongoDB from JavaScript file - mongodb

Question

I'm trying to write a MongoDB script as part of automating customer data deletion in accordance with GDPR (current process is manual), but I'm having problems authenticating from the script. Authentication works perfectly from the command line, but not from a script.
I'm not certain which mechanism I should use, but since there are only two that don't require external files (which I don't use in the command line), I figured I'd try both.
When running from the command line, this is the command I use:
$ mongo -u [adminusr] -p '[adminpwd]' --authenticationDatabase admin
This script:
var host = '127.0.0.1';
var port = '27017';
var user = 'admin';
var pwd = 'secrets';
var authDB = 'admin';
try {
var conn = new Mongo('mongodb://' + host + ':' + port + '/' + authDB);
conn.auth({
user: user,
pwd: pwd,
mechanism: 'SCRAM-SHA-1',
digest: false
});
// deletion commands...
} catch (e) {
print(e);
}
gives this error:
$ mongo erasure.js
MongoDB shell version v3.4.2
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.10
Error: Authentication failed.
Changing the auth part to this:
conn.auth({
user: user,
pwd: pwd,
mechanism: 'MONGODB-CR',
db: authDB,
digest: false
});
gives me this error:
$ mongo erasure.js
MongoDB shell version v3.4.2
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.10
Error: auth failed
The official documentation on writing MongoDB scripts and authentication is also very scarce on how to do this.
How can I get the auth to work?
Thanks.