how to replace ObjectId in JavaScript with Epoch Time

How do I iterate through a JSON string and replace every ObjectId into Unix Epoch time for further processing?

What I know: You get the first 8 chars from an objectId with:

subStrObjectId = objectId.substring(0, 8);

5668d142a54cc4960b55ea19 --> 5668D142

and convert these from hexadecimal into an Int value (epoch time in milliseconds):

subStrObjectIdInDec = parseInt(subStrObjectId, 16);

5668D142 (hex) --> 1449709890 (dec)

my Json string:

myJsonString = [
    [
        {"_id":"5668d142a54cc4960b55ea19","cid":10851045,"temp":25.4},
        {"_id":"5668d14ea54cc4960b55ea1a","cid":10850909,"temp":24.9}
    ],
    [
        {"_id":"5668d14fa54cc4960b55ea1b","cid":10851045,"hum":37.9},
        {"_id":"5668d3108c8cda92074b7ec9","cid":10850909,"hum":39.6}
    ],
    [
        {"_id":"5668d3198c8cda92074b7ecb","cid":10851045,"lux":34},
        {"_id":"5668d31e8c8cda92074b7ecc","cid":10850909,"lux":68}
    ]
];

If you write what you know as a function:

function convert(id) {
  return parseInt(id.substring(0, 8), 16);
}

You can easily iterate over your objects and run your function on the objects.

I'd prefer functional-style javascript:

var data = myJsonString.map(function (array) {
  return array.map(function (item) {
    item.time = convert(item._id);
    return item;
  })
})

console.log(data);

But you can go iterative with loops as well:

for (var i=0;i<myJsonString.length;i++) {
  for (var j=0;j<myJsonString[i].length;j++) {
    myJsonString[i][j].time = convert(myJsonString[i][j]._id);
  }
}
console.log(myJsonString);