Using do-while loop in NodeJS to check existed in Firebase database

Question

When a user registers a new account, I want to generate a reference code. I'm using the do-while loop for checking exists of the reference code in the database but the loop keeps running at least 2 times although the records in the database are small. When I tried to run the code, it was stuck and keep waiting for the response forever.

Here's my implement:

function makeCode() {
    var text = "";
    var possible = "ABCDEFGHIJKLMNPQRSTUVWXYZabcdefghijklmnpqrstuvwxyz0123456789"; 
    for (var i = 0; i < 8; i++) {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }
    console.log("### makeReferenceCode DONE ###")
    return text;
}

function genReferenceCode() {
    let referencePromise = new Promise((resolve, reject) => {
        let referenceRef = db.ref("ref_codes")
        var isExisted = true
        var code = makeCode().toUpperCase()
        console.log("### Generate Reference Code: " + code)
        do {
            referenceRef.orderByChild("code").equalTo(code).limitToFirst(1).on("value", function (snapshots) {
                if(snapshots.numChildren() > 0) {
                    console.log("### code existed ###")
                    code = makeCode().toUpperCase()
                    isExisted = true 
                } else {
                    console.log("### code not existed: " + code)
                    isExisted = false 
                    resolve({success: true, data: code})
                }
            }, function (error) {
                isExisted = true
                reject({success: false, error_code: "GEN_REF_CODE_00", error_msg: "Can't generate reference code!"})
            })
        } while (isExisted === true)
    })
    return referencePromise;
}