Streams are buffered and emit data
events whenever they please (so to speak), not on strict boundaries like lines of text.
But you can use the readline
module to parse the buffers into lines for you:
var child_process = require('child_process');
var readline = require('readline');
var proc = child_process.spawn(...);
readline.createInterface({
input : proc.stdout,
terminal : false
}).on('line', function(line) {
console.log(line);
});