Assign JSON value to variable based on value of a different key - json

Question

I have this function for extracting the timestamp from two JSON objects:
lineReader.on('line', function (line) {
var obj = JSON.parse(line);
if(obj.Event == "SparkListenerApplicationStart" || obj.Event == "SparkListenerApplicationEnd") {
console.log('Line from file:', obj.Timestamp);
}
});
The JSON comes from a log file(not JSON) where each line represents an entry in the log and each line also happens to be in JSON format on its own.
The two objects represent the start and finish of a job. These can be identified by the event key(SparkListenerApplicationStart and SparkListenerApplicationEnd). They also both contain a timestamp key. I want to subtract the end time from the start time to get the duration.
My thinking is to assign the timestamp from the JSON where Event key = SparkListenerApplicationStart to one variable and assign the timestamp from the JSON where Event key = SparkListenerApplicationEnd to another variable and subtract one from the other. How can I do this? I know I can't simply do anything like:
var startTime = if(obj.Event == "SparkListenerApplicationStart"){
return obj.Timestamp;
}

Answer 1

I'm not sure if I understood, but if are reading rows and want get the Timestamp of each row I would re-write a new object;
const collection = []
lineReader.on('line', function (line) {
var obj = JSON.parse(line);
if(obj.Event == "SparkListenerApplicationStart" || obj.Event == "SparkListenerApplicationEnd") {
// console.log('Line from file:', obj.Timestamp);
collection.push(obj.Timestamp)
}
});
console.log(collection);
Where collection could be a LocalStorage, Global Variable, or something alike.
Additional info
With regard to my comment where I queried how to identify the start and end times, I ended up setting start as the smallest value and end as the largest. Here is my final code:
const collection = []
lineReader.on('line', function (line) {
var obj = JSON.parse(line);
if((obj.Event == "SparkListenerApplicationStart" || obj.Event == "SparkListenerApplicationEnd")) {
collection.push(obj.Timestamp);
if(collection.length == 2){
startTime = Math.min.apply(null, collection);
finishTime = Math.max.apply(null, collection);
duration = finishTime - startTime;
console.log(duration);
}
}
});