$ node -v
v4.2.6

$ node debug format.js 
< Debugger listening on port 5858
debug> . ok
break in format.js:1
> 1 var util = require('util');
  2 var output = util.format({ p1: 'v1' });
  3 console.log(output);
debug> n
break in format.js:2
  1 var util = require('util');
> 2 var output = util.format({ p1: 'v1' });
  3 console.log(output);
  4 
debug> n
program terminated