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JavaScript

const arr = {
 field1: { data: "C", index: 3 },
 field2: { data: "D", index: 4 },
 field3: { data: "B", index: 2 },
 field4: { data: "A", index: 1 },
};

const data = Object.entries(arr)

const sortedDataArray = data.sort((a, b) => a[1].index - b[1].index)

const result = Object.fromEntries(sortedDataArray)


console.log(result);