Whether you're doing this in the browser or in Node, you should be able to pass the JSON string to JSON.parse and pick out the value you want. You can then append to a file using Node's fs module like this: https://stackoverflow.com/a/11267583/659910.
fs = require 'fs'
# Sample JSON string.
json = '{ "statType": "TOTAL_SESSIONS_PLAYED", "count": { "value": 5 }, "dataVersion": 0 }'
data = JSON.parse(json)
fs.appendFile('/tmp/data.txt', data.count.value, (error) -> throw error if error)
