@babel/plugin-transform-parameters
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
function test(x = "hello", { a, b }, ...args) {
console.log(x, a, b, args);
}
Out
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
- Yarn
- pnpm
npm install --save-dev @babel/plugin-transform-parameters
yarn add --dev @babel/plugin-transform-parameters
pnpm add --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
With a configuration file (Recommended)
{
"plugins": ["@babel/plugin-transform-parameters"]
}
Via CLI
babel --plugins @babel/plugin-transform-parameters script.js
Via Node API
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.
{
"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.
// 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