Jsonのデータをループさせて特定の値を取り出したい

期間中のhighの最大値とlowの最小値は次のようにして取れます。

var partialData = [
    { "high": "10", "low": "1" },
    { "high": "20", "low": "2" },
    { "high": "30", "low": "3" },
    { "high": "40", "low": "4" },
    { "high": "50", "low": "5" },
    { "high": "60", "low": "6" }
]

var highNumbers = partialData.map(data => parseInt(data.high))
// [ 10, 20, 30, 40, 50, 60 ]
var highest = Math.max.apply(null, highNumbers)
// 60

var lowNumbers = partialData.map(data => parseInt(data.low))
// [ 1, 2, 3, 4, 5, 6 ]
var lowest = Math.min.apply(null, lowNumbers)
// 1

それよりも気がかりなのが、貼られたコードで区間に区切れていないのではないでしょうか。

for (var i = 0; i < json.length; i += 7) {
    fs.appendFile('./out.json', JSON.stringify(json[i], null, '    '));
}

これだと「7日分を一区間とした形にデータを分割する」のではなく、「7日とばしに1日ずつサンプリングする」という実装になっています。

JSON.stringify してファイルに書き込むところは一旦置いておくとして、こんな感じですかね。

ここ で実行できます。

 実行結果

 サンプルコード

const data =  [
    {
        "day": "2018/04/30",
        "high": "1039537",
        "low": "1004541"
    },
    {
        "day": "2018/04/29",
        "high": "1048158",
        "low": "1000610"
    },
    {
        "day": "2018/04/28",
        "high": "1042000",
        "low": "982147"
    },
    {
        "day": "2018/04/27",
        "high": "1044772",
        "low": "960676"
    },
    {
        "day": "2018/04/26",
        "high": "1071110",
        "low": "960690"
    },
    {
        "day": "2018/04/25",
        "high": "1076648",
        "low": "967815"
    },
    {
        "day": "2018/04/24",
        "high": "1067674",
        "low": "964904"
    },
    {
        "day": "2018/04/23",
        "high": "988147",
        "low": "947067"
    },
    {
        "day": "2018/04/22",
        "high": "978223",
        "low": "935823"
    },
    {
        "day": "2018/04/21",
        "high": "974495",
        "low": "891117"
    },
    {
        "day": "2018/04/20",
        "high": "963122",
        "low": "870957"
    },
    {
        "day": "2018/04/19",
        "high": "896137",
        "low": "853827"
    },
    {
        "day": "2018/04/18",
        "high": "880135",
        "low": "841612"
    },
    {
        "day": "2018/04/17",
        "high": "898900",
        "low": "841899"
    },
    {
        "day": "2018/04/16",
        "high": "906178",
        "low": "845470"
    },
    {
        "day": "2018/04/15",
        "high": "898839",
        "low": "846863"
    },
    {
        "day": "2018/04/14",
        "high": "875603",
        "low": "827362"
    },
    {
        "day": "2018/04/13",
        "high": "878800",
        "low": "725550"
    },
    {
        "day": "2018/04/12",
        "high": "854537",
        "low": "721461"
    },
    {
        "day": "2018/04/11",
        "high": "744774",
        "low": "712095"
    },
    {
        "day": "2018/04/10",
        "high": "778178",
        "low": "711808"
    },
    {
        "day": "2018/04/09",
        "high": "780201",
        "low": "716405"
    },
    {
        "day": "2018/04/08",
        "high": "769416",
        "low": "732451"
    },
    {
        "day": "2018/04/07",
        "high": "761432",
        "low": "699899"
    },
    {
        "day": "2018/04/06",
        "high": "746511",
        "low": "699204"
    },
    {
        "day": "2018/04/05",
        "high": "789659",
        "low": "708030"
    },
    {
        "day": "2018/04/04",
        "high": "801088",
        "low": "720637"
    },
    {
        "day": "2018/04/03",
        "high": "802688",
        "low": "723628"
    },
    {
        "day": "2018/04/02",
        "high": "750759",
        "low": "678044"
    },
    {
        "day": "2018/04/01",
        "high": "770192",
        "low": "676906"
    }
];

const weeks = [];

for (i = 0; i < data.length; i += 7) {
  const week = data.slice(i, 7 + i);
  weeks.push(week);
}

console.log(weeks);

const weeksSummary = weeks.map(week => ({
  maxHigh: Math.max(...week.map(data => data.high)),
  minLow: Math.min(...week.map(data => data.low)),
}));

console.log(weeksSummary);

weeksSummary.forEach((summary, weekNumber) => {
  const message = `第${weekNumber + 1}週目  最大値: ${summary.maxHigh}, 最小値: ${summary.minLow}`;

  console.log(message);
});