How to use the flatbuffers.flatbuffers.Long function in flatbuffers

function buildHeader(features: any, header: HeaderMeta) {
    const length = features.length
    const builder = new flatbuffers.Builder(0)

    let columnOffsets = null
    if (header.columns)
        columnOffsets = Header.createColumnsVector(builder,
            header.columns.map(c => buildColumn(builder, c)))

    const nameOffset = builder.createString('L1')

    Header.start(builder)
    Header.addFeaturesCount(builder, new flatbuffers.Long(length, 0))
    Header.addGeometryType(builder, header.geometryType)
    Header.addIndexNodeSize(builder, 0)
    if (columnOffsets)
        Header.addColumns(builder, columnOffsets)
    Header.addName(builder, nameOffset)
    const offset = Header.end(builder)
    builder.finishSizePrefixed(offset)
    return builder.asUint8Array()
}

function buildHeader(featurecollection: IGeoJsonFeatureCollection, header: HeaderMeta) {
    const length = featurecollection.features.length
    const builder = new flatbuffers.Builder(0)

    let columnOffsets = null
    if (header.columns)
        columnOffsets = Header.createColumnsVector(builder,
            header.columns.map(c => buildColumn(builder, c)))

    const nameOffset = builder.createString('L1')

    Header.start(builder)
    Header.addFeaturesCount(builder, new flatbuffers.Long(length, 0))
    Header.addGeometryType(builder, header.geometryType)
    Header.addIndexNodeSize(builder, 0)
    if (columnOffsets)
        Header.addColumns(builder, columnOffsets)
    Header.addName(builder, nameOffset)
    const offset = Header.end(builder)
    builder.finishSizePrefixed(offset)
    return builder.asUint8Array()
}