var merge = require('merge-source-map')

var esprima    = require('esprima'),
    estraverse = require('estraverse'),
    escodegen  = require('escodegen'),
    convert    = require('convert-source-map'),
    merge      = require('merge-source-map')

const CODE = 'a = 1',
      FILEPATH = 'a.js'

// create AST of original code
var ast = esprima.parse(CODE, {sourceType: 'module', loc: true})

// transform AST of original code
estraverse.replace(ast, {
  enter: function(node, parent) { /* change AST */ },
  leave: function(node, parent) { /* change AST */ }
})

// generate code and source map from transformed AST
var gen = escodegen.generate(ast, {
  sourceMap: FILEPATH,
  sourceMapWithCode: true,
  sourceContent: CODE
})

// merge old source map and new source map
var oldMap = convert.fromSource(CODE) && convert.fromSource(CODE).toObject(),
    newMap = JSON.parse(gen.map.toString()),
    mergedMap = merge(oldMap, newMap),
    mapComment = convert.fromObject(mergedMap).toComment()

// attach merge source map to transformed code
var transformed = gen.code + '\n' + mapComment

console.log(transformed);

% npm install
% npm test