foo.js

var parse = require('esprima').parse;
var generate = require('escodegen').generate;


function foo() {
  var a = 1;
  (function () {
    var b = 2;
    console.log(a);
  })();
  console.log(a);
  console.log(b);
}

function body(fn) {
  var ast = parse(foo).body[0].body;
  var gen = generate(ast, { format: { compact: true } });
  return gen.substring(1, gen.length - 1);
}

console.log(body(foo));
//=> var a=1;(function(){var b=2;console.log(a);}());console.log(a);console.log(b);