可以将对象放在数组中，只需将当前索引存储在另一个变量中。我还添加了made，这样它就可以循环，如果您需要的话，只需删除ternaries并使用 Math.max 及 Math.min 相反

var index = 0;

function next() {
  index = ++index >= phases.length ? 0 : index;
  console.log(phases[index].status);
}

function prev() {
  index = --index <= 0 ? phases.length - 1 : index;
  console.log(phases[index].status);
}

var phases = [{
  startDate: "2021-06-26T11:00:00.000Z",
  endDate: "2030-12-31T09:00:00.000Z",
  status: "ended"
}, {
  startDate: "1899-12-31T21:00:00.000Z",
  endDate: "2021-06-17T09:25:31.000Z",
  status: "openingSoon"
}, {
  startDate: "2021-06-15T21:00:00.000Z",
  endDate: "2021-08-30T21:00:00.000Z",
  status: "playing"
}, {
  startDate: "2021-06-13T11:00:00.000Z",
  endDate: "2021-06-15T21:00:00.000Z",
  status: "readyToPlay"
}, {
  startDate: "2021-06-07T19:00:00.000Z",
  endDate: "2021-06-12T17:00:00.000Z",
  status: "registrationsOpen"
}, {
  startDate: "2021-08-30T21:00:00.000Z",
  endDate: "2021-06-26T11:00:00.000Z",
  status: "resultsPending"
}];

<button onclick="prev()">Previous</button>
<button onclick="next()">Next</button>

编辑fast es6将原始对象转换为阵列的方法：

Object.entries(phases).map(([key, value]) => ({ ...value, status: key }));

ie兼容方法：

Object.keys(phases).map(function (key) {
    var oldObject = {}, newObject = {};
    for (var subKey in oldObject) newObject[subKey] = oldObject[subKey];
    newObject.status = key;
    return newObject;
});

const phasesStatuses = Object.keys(phases); // Get array of phases statuses
const currentPhaseIndex = phasesStatuses.indexOf(currentPhase.status); // get index of current phase

const nextPhaseStatus = phasesStatuses[currentPhaseIndex + 1]; // get next phase status, which will be the key of next phase in phases array
const nextPhase = phasesStatuses[nextPhaseStatus];

const currentPhase = {
    endDate: "2030-12-31T09:00:00.000Z",
    startDate: "2021-06-26T11:00:00.000Z",
    status: "ended",
}

const phases = {
    ended: {
        startDate: "2021-06-26T11:00:00.000Z",
        endDate: "2030-12-31T09:00:00.000Z" },
    openingSoon: {
        startDate: "1899-12-31T21:00:00.000Z",
        endDate: "2021-06-17T09:25:31.000Z" },
    playing: {
        startDate: "2021-06-15T21:00:00.000Z",
        endDate: "2021-08-30T21:00:00.000Z" },
    readyToPlay: {
        startDate: "2021-06-13T11:00:00.000Z",
        endDate: "2021-06-15T21:00:00.000Z" },

    registrationsOpen: {
        startDate: "2021-06-07T19:00:00.000Z",
        endDate: "2021-06-12T17:00:00.000Z" },

    resultsPending: {
        startDate: "2021-08-30T21:00:00.000Z",
        endDate: "2021-06-26T11:00:00.000Z" },
}

const phasesList = Object.keys(phases)
const currentPhaseIndex = phasesList.indexOf(currentPhase.status)
const nextPhase = currentPhaseIndex === phasesList.length - 1 ? phasesList[0] : phasesList[currentPhaseIndex + 1]

console.log(nextPhase)