Javascript - 如何解析 JSON 文件以从最近的条目中查找单个数据点

Question

I am trying to measure temperature using a sensor which is then uploaded to a mongo database. This is then retrieved as a JSON file, accessable from visiting ('/data').

I would like to firstly find the most recent entry, then find the value of the temperature from that.

This is how the JSON looks (x10,000):

{
"_id": {
    "$oid": "5882d98abd966f163e36a6cf"
},
"temperature": 19,
"humidity": 30,
"epochtime": 1484970377120
}

I have set up the website using express.js, I believe I will have to somehow parse this JSON object, maybe using this to step through and find the most recent entry.

for ("temperature" in '/data') {
    output += temperature + ': ' + obj[temperature]+'; ';
}
console.log(output[0]);

Many thanks for the assistance.

---

UPDATE & Solution:

Many thanks to Bertrand Martel, almost perfect solution.

MongoClient.connect(url, function(err, db) {
assert.equal(null, err);
console.log("Connected successfully to server");

db.collection('Sensors', function(err, collection){
assert.equal(err, null);
var options = {
  "limit": 1
}
var sort = {
  "epochtime": -1
}

collection.find({}, options).sort(sort).toArray(function(err, res) {
assert.equal(err, null);

console.log("most recent temperature : " + res[0].temperature);
console.log("most recent humidity : " + res[0].humidity);
    }); 
});
db.close();
});

Answer 1

You can sort epochtime backward and limit the result to 1 document :

If you use node-mongodb-native :

var MongoClient = require('mongodb').MongoClient;

MongoClient.connect("mongodb://localhost:27017/testDB", function(err, db) {
    if (err) {
        return console.dir(err);
    }

    db.collection('test', function(err, collection) {

        collection.find({}, { "limit": 1 }).sort({ "epochtime": -1 }).toArray(function(err, res) {
            console.log("most recent temperature : " + res[0].temperature);
        });

    });
});