rm-merge.js

module.exports = function(file, api) {
  var j = api.jscodeshift; // alias the jscodeshift API
  var root = j(file.source); // parse JS code into an AST

  // the main update method replaces merge with ObjectExpression
  const update = path =>
    j(path).replaceWith(j.objectExpression(
      flatten(path.value.arguments.map(p =>
        p.type == 'ObjectExpression'
          ? p.properties
          : j.spreadProperty(p)
      ))
    ));

  // find and update all merge calls
  root.find(j.CallExpression, {callee: {name: 'merge'}})
   .forEach(update);

  // print
  return root.toSource();
};

// A lovely recursive helper
const flatten =
  a => Array.isArray(a) ? [].concat(...a.map(flatten)) : a;