JavaScript – reduce the valuses of a specific object field from an array of objects to a single string [closed]

I have an array of insurance policy objects. The policy object has several fields, one of which is policyNumber. I am trying to create a string of the policy numbers using JavaScript.

My code thus far is as follows:

const policies = [
    {
        name: "betty",
        policyNumber: "12345",
        policyKindCode: "ABC"
    },
    {
        name: "debbie",
        policyNumber: "67890",
        policyKindCode: "DEF"
    },
    {
        name: "judy",
        policyNumber: "13579",
        policyKindCode: "GHI"
    },
    {
        name: "rudy",
        policyNumber: "24680",
        policyKindCode: "JKL"
    }
  ] 

  const policiesString = policies.reduce((str, policy) => {
    str += (policy.policyNumber + ", ");
    return str;
  });

  console.log('policiesString = ' + policiesString);

The console.log is returning the following:

"policiesString = [object Object]67890, 13579, 24680, "

If I comment out the first object it just puts the [object Object] where 67890 would go and then has the last two policy numbers, and so on.

What am I doing wrong?