JSFiddle - React, Tailwind, and code Playground

CSS

body {
    white-space: pre;
}

JavaScript

var data = [
    {"Username": "Mehmet", "UserID": 2, "OkeyTablePlayerChairNumber": 1},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 2},
    {"Username": "Erçin", "UserID": 1, "OkeyTablePlayerChairNumber": 3},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 4},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 5},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 6},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 7},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 8},
    {"Username": null, "UserID": 0, "OkeyTablePlayerChairNumber": 9}
];

    var wantedId = 1;

    var index = 0, result;
    //int yourSitPositionIndex = playersOnTheTableWithEmpytPositions.ToList().FindIndex(x => x.UserID == userID);
    while (index < data.length && data[index].UserID != wantedId) index += 1;
    if (index < data.length) {
        //var beforePlayers = playersOnTheTableWithEmpytPositions.ToList().GetRange(0, yourSitPositionIndex);
        var beforePlayers = data.slice(0, index);
        //IEnumerable<tbl_Okey_TablePlayer> afterPlayers = playersOnTheTableWithEmpytPositions.Except(beforePlayers);
        var afterPlayers = data.slice(index);
        //IEnumerable<tbl_Okey_TablePlayer> newPositions = afterPlayers.Concat(beforePlayers);
        result = afterPlayers.concat(beforePlayers);
    } else {
        result = data;
    }


document.body.innerText = result.map(JSON.stringify).join('\n');