Creating lists with section headers in React Native

section_headers.jsx

let timestamps = [
    { timestamp: Date.now(), period: Periods.ONE },
    { timestamp: Date.now(), period: Periods.ONE },
    { timestamp: Date.now(), period: Periods.ONE },
    { timestamp: Date.now(), period: Periods.THREE },
    { timestamp: Date.now(), period: Periods.FOUR }
];

function createTimestampMap(timestamps) {

    let categoryMap = {};

    Object.keys(Periods).forEach((key) => {
        let period = Periods[key];
        categoryMap[period.label] = [];
    });

    timestamps.forEach((timestamp) => {
        categoryMap[timestamp.period.label].push(timestamp);
    });

    return categoryMap;

}

let timestampMap = createTimestampMap(timestamps);
let sectionTemplate = new ListView.DataSource({
    rowHasChanged: (r1, r2) => r1 !== r2,
    sectionHeaderHasChanged: (s1, s2) => s1 !== s2
});