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@babel/plugin-transform-parameters

NOTE: This plugin is included in @babel/preset-env

This plugin transforms ES2015 parameters to ES5, this includes:

  • Destructuring parameters
  • Default parameters
  • Rest parameters

Examples​

In

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

Out

JavaScript
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;
    _key++
  ) {
    args[_key - 2] = arguments[_key];
  }

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

Installation​

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

Caveats​

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.

Usage​

babel.config.json
{
  "plugins": ["@babel/plugin-transform-parameters"]
}

Via CLI​

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

Via Node API​

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

Options​

loose​

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.

⚠️ Consider migrating to the top level ignoreFunctionLength assumption.

babel.config.json
{
  "assumptions": {
    "ignoreFunctionLength": true
  }
}

Under the ignoreFunctionLength assumption, Babel will generate 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.

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

// ignoreFunctionLength: true
function bar1(arg1 = 1) {}
bar1.length; // 1

You can read more about configuring plugin options here