gistfile1.txt

$cat parse-json.js 
#!/usr/bin/env node

var readline = require('readline');

var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  terminal: false
});

rl.on('line', function(line) {
  var data = JSON.parse(line);
  data.new_field = 3;
  console.log(JSON.stringify(data));
});

$ cat parse-json.py
#!/usr/bin/env python

import json
import sys

for line in sys.stdin:
    data = json.loads(line)
    data['new_field'] = 3
    print(json.dumps(data))

$ wc enron-events-sentiment.json 
    518147  221392944 2091624861 enron-events-sentiment.json

$ time ./parse-json.js < enron-events-sentiment.json > tmp.json

real	0m40.884s
user	0m36.226s
sys	0m4.636s

$ time ./parse-json.py < enron-events-sentiment.json > tmp.json

real	0m52.010s
user	0m47.724s
sys	0m3.469s

$cat cat.js
#!/usr/bin/env node

var readline = require('readline');

var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  terminal: false
});

rl.on('line', function(line) {
  console.log(line);
});