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Something like this? And then change length and possible to the desired rng length and complexity.

function rng(){
    let uID = '';
        let length = 8;
    let possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    for (let i = 0; i < length; i++) {
        uID += possible.charAt(Math.floor(Math.random() * possible.length));
    }
    return uID;
}