Converts all Object.assign calls to { ... obj } spread.

object-assign-to-spread.js

// Highly based on an example from https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb

module.exports = function(file, api) {
  var j = api.jscodeshift;
  var root = j(file.source);

  const update = path =>
    j(path).replaceWith(j.objectExpression(
      flatten(path.value.arguments.map(p =>
        p.type === 'ObjectExpression'
          ? p.properties
          : j.spreadProperty(p)
      ))
    ));

  const call = root.find(j.CallExpression, {
    callee: {
      object: {
        name: 'Object'
      },
      property: {
      	name: 'assign'
      }
    }})
   .forEach(update);

  return root.toSource({
    trailingComma: true,
    tabWidth: 2,
    quote: 'single',
  });
};

const flatten =
  a => Array.isArray(a) ? [].concat(...a.map(flatten)) : a;