df0d9d05a3
Similar in spirit to https://github.com/babel/babel/pull/6580, the current implementation did ```js node.params[node.params.length - 1] ``` where `node.params` can also be empty, which causes it to lookup the property `"-1"`, which is not found on the array itself and obviously also not in the `Object.prototype` and the `Array.prototype`. However since `"-1"` is not a valid array index, but has a valid integer representation, this is a very expensive lookup in V8 (and probably other engines too, but that is probably less relevant, since Babel most often runs on Node nowadays). In V8 this causes a call to the `%SetProperty` runtime function for each of these `"-1"` property lookups, and in addition sends the whole `KeyedLoadIC` to `MEGAMORPHIC` state, which also penalizes other accesses on this line. This is a small non-breaking performance fix.
@babel/plugin-transform-es2015-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-es2015-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
transform-block-scoping plugin.
Usage
Via .babelrc (Recommended)
.babelrc
{
"plugins": ["@babel/transform-es2015-parameters"]
}
Via CLI
babel --plugins @babel/transform-es2015-parameters script.js
Via Node API
require("@babel/core").transform("code", {
plugins: ["@babel/transform-es2015-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