1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
|
# @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**
```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
```sh
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](http://babeljs.io/docs/plugins/transform-block-scoping) plugin.
## Usage
### Via `.babelrc` (Recommended)
**.babelrc**
```json
{
"plugins": ["@babel/plugin-transform-parameters"]
}
```
### Via CLI
```sh
babel --plugins @babel/plugin-transform-parameters script.js
```
### Via Node API
```javascript
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.
```javascript
// Spec behavior
function bar1 (arg1 = 1) {}
bar1.length // 0
// Loose mode
function bar1 (arg1 = 1) {}
bar1.length // 1
```
|
