Skip to content
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
106 lines (76 sloc) 2.14 KB
id: babel-plugin-transform-parameters
title: @babel/plugin-transform-parameters
sidebar_label: transform-parameters

This plugin transforms ES2015 parameters to ES5, this includes:

  • Destructuring parameters
  • Default parameters
  • Rest parameters



function test(x = "hello", { a, b }, ...args) {
  console.log(x, a, b, args);


function test() {
  var x =
    arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : "hello";
  var _ref = arguments[1];
  var a = _ref.a,
    b = _ref.b;

  for (
    var _len = arguments.length,
      args = Array(_len > 2 ? _len - 2 : 0),
      _key = 2;
    _key < _len;
  ) {
    args[_key - 2] = arguments[_key];

  console.log(x, a, b, args);


npm install --save-dev @babel/plugin-transform-parameters


Default parameters desugar into let declarations to retain proper semantics. If this is not supported in your environment then you'll need the @babel/plugin-transform-block-scoping plugin.


Via .babelrc (Recommended)


  "plugins": ["@babel/plugin-transform-parameters"]


babel --plugins @babel/plugin-transform-parameters script.js

Via Node API

require("@babel/core").transform("code", {
  plugins: ["@babel/plugin-transform-parameters"],



boolean, defaults to false.

In loose mode, parameters with default values will be counted into the arity of the function. This is not spec behavior where these parameters do not add to function arity.

The loose implementation is a more performant solution as JavaScript engines will fully optimize a function that doesn't reference arguments. Please do your own benchmarking and determine if this option is the right fit for your application.

// Spec behavior
function bar1(arg1 = 1) {}
bar1.length; // 0

// Loose mode
function bar1(arg1 = 1) {}
bar1.length; // 1

You can read more about configuring plugin options here

You can’t perform that action at this time.