Converts underscore/lodash `.each()` with `this` context to use arrow functions.

each-with-this-to-arrows.js

/**
 * Converts underscore/lodash `.each()` with `this` context to use arrow functions.
 *
 * Run this with jscodeshift
 * Live demo: https://astexplorer.net/#/gist/0a47495d69719449d2afbb0f0c50f8ea/latest
 *
 * Converts:
 *     _.each(array, function(item) {
 *       // ...
 *     }, this);
 *
 * to:
 *     _.each(array, item => {
 *       // ...
 *     });
 */

module.exports = function transformer(file, api) {
  const j = api.jscodeshift;

  return j(file.source)
    .find(j.CallExpression)
    .filter(
      path =>
        path.node.callee.type === "MemberExpression" &&
        path.node.callee.object.type === "Identifier" &&
        path.node.callee.object.name === "_" &&
        path.node.callee.property.type === "Identifier" &&
        path.node.callee.property.name === "each" &&
        path.node.arguments.length === 3 &&
        path.node.arguments[1].type === "FunctionExpression" &&
        path.node.arguments[2].type === "ThisExpression"
    )
    .forEach(path => {
      let iterator = path.node.arguments[1];
      path.node.arguments[1] = j.arrowFunctionExpression(iterator.params, iterator.body, iterator.expression);
      path.node.arguments.length = 2;
    })
    .toSource();
};