f8ab9466d3
We were using `Object.create` to setup the prototype chain at the start of the class definition, which lead to #7771. I was a bit worried about a speed hit, but it seems everyone optimizes the two patterns the same way. https://jsbench.github.io/#f9fca52407643d96458a35763b201215 Fixes #7771.
@babel/plugin-transform-parameters
Compile ES2015 default and rest parameters to ES5
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 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
Via .babelrc (Recommended)
.babelrc
{
"plugins": ["@babel/plugin-transform-parameters"]
}
Via CLI
babel --plugins @babel/plugin-transform-parameters script.js
Via Node API
require("@babel/core").transform("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.
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