gistfile1.js

function doYourStuff() {
    const yourString = "foobar"
    const uniqueString = findUniqueString(yourString)

    // continue with your logic with unique string
}

function findUniqueString(baseString) {
    let uniqueString = baseString
    // number of iterations should be super small, probably unnecessary to have a max threshold
    // it would be smarter to not have this as unique identifier and have an additional field that
    // is an auto-increment and have a composite key or something else
    let iteration = 0
    while(true) {
        if (!existsInDatabase(uniqueString)) {
            return uniqueString
        } else {
            iteration++
            uniqueString = `${baseString}_${iteration}`
        }
    }
}

function existsInDatabase(str) {
    return // <some query>
}

Thanks Kevin!