const currencyName = ["btc", "eth", "xrp"];
let dataCollections = [];
for (i = 0; i < currencyName.length; i ++) {
  dataCollections.push(new Promise(function(resolve, reject) {
    request.get(`/${currencyName[i]}jpy`, function(error, response, body) {
      if (error) {
        reject(error);
      } if (dataCollections.length === 3) {
        dataColletions = [];
      }  else {
        resolve(JSON.parse(body));
      }
    });
  }));
}

//Promise.all(??).then(??)

//For rendering dataCollections
  app.get("/", function(req, res) {
    res.render("home", {dataCollections: dataCollections});
}

回答如下：

1）考虑将数组currencyNames命名为复数，以便您可以为迭代器currencyName命名。

2）您的for循环正在重新发明array.map函数。考虑改用array.map。

3）您的for循环有一个不可能的if子句（if (dataCollections.length === 3)），这也意味着else子句在错误的if语句上。我认为这是一个错字。

现在是您的问题的核心，是Proimse.all(Array<Promise>)是您想要的；当其参数中的所有元素都解析后，它就会解析。

const currencyNames = ["btc", "eth", "xrp"];

let dataCollections = currencyNames.map(currencyName => new Promise((resolve, reject) =>
    request.get(`https://apiv2.bitcoinaverage/indices/global/ticker/${currencyName}jpy`, (error, response, body) => {
      if (error)
        reject(error);
      else
        resolve(JSON.parse(body));
    })));

//For rendering dataCollections
app.get("/", async (req, res) =>
    res.render("home", {dataCollections: await Promise.all(dataCollections)}));