output.json

{
  "type": "Program",
  "elements": [
    {
      "type": "VariableStatement",
      "declarations": [
        {
          "type": "VariableDeclaration",
          "name": "a",
          "value": {
            "type": "BinaryExpression",
            "operator": "+",
            "left": {
              "type": "NumericLiteral",
              "value": 1
            },
            "right": {
              "type": "NumericLiteral",
              "value": 2
            }
          }
        }
      ]
    }
  ]
}

run.js

util = require('util'),
fs = require('fs'),
peg = require('./peg')

file = fs.readFileSync('ecmascript.pegjs','utf-8')
parser = peg.buildParser(file)
ast = parser.parse('var a = 1+2')
json = JSON.stringify(ast, null, '  ')
util.puts(json)