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# Babel Legacy Decorator plugin
This is a plugin for Babel 6 that is meant to replicate the old decorator behavior from
Babel 5 in order to allow people to more easily transition to Babel 6 without needing to
be blocked on updates to the decorator proposal or for Babel to re-implement it.
## Babel >= 7.x
This plugin is specifically for Babel 6.x. If you're using Babel 7, this plugin is not for you.
Babel 7's `@babel/plugin-proposal-decorators` officially supports the same logic that this
plugin has, but integrates better with Babel 7's other plugins. You can enable this with
```json
{
"plugins": [
["@babel/plugin-proposal-decorators", { "legacy": true }],
]
}
```
in your Babel configuration. Note that `legacy: true` is specifically needed if you
want to get the same behavior as `transform-decorators-legacy` because there
are newer versions of the decorator specification coming out, and they do not
behave the same way as this plugin does.
## Installation & Usage
$ npm install --save-dev babel-plugin-transform-decorators-legacy
Add the following line to your .babelrc file:
{
"plugins": ["transform-decorators-legacy"]
}
#### NOTE: Order of Plugins Matters!
If you are including your plugins manually and using `transform-class-properties`, make sure that `transform-decorators-legacy` comes *before* `transform-class-properties`.
```js
/// WRONG
"plugins": [
"transform-class-properties",
"transform-decorators-legacy"
]
// RIGHT
"plugins": [
"transform-decorators-legacy",
"transform-class-properties"
]
```
## Why "legacy"?
Decorators are still only a relatively new proposal, and they are (at least currently) still
in flux. Many people have started to use them in their original form, where each decorator
is essentially a function of the form
function(target, property, descriptor){}
This form is very likely to change moving forward, and Babel 6 did not wish to support
the older form when it was known that it would change in the future. As such, I created this
plugin to help people transition to Babel 6 without requiring them to drop their decorators
or requiring them to wait for the new proposal update and then update all their code.
## Best Effort
This plugin is a best effort to be compatible with Babel 5's transpiler output, but there
are a few things that were difficult to reproduce, and a few things that were simply incorrect
in Babel 5 with respect to the decorators proposal.
Two main things to mention as differences, though not things you are likely to encounter:
1. Decorators expressions are evaluated top to bottom, and executed bottom to top. e.g.
```
function dec(id){
console.log('evaluated', id);
return (target, property, descriptor) => console.log('executed', id);
}
class Example {
@dec(1)
@dec(2)
method(){}
}
```
In Babel 5, this would output:
```
evaluated 2
evaluated 1
executed 2
executed 1
```
With this plugin, it will result in:
```
evaluated 1
evaluated 2
executed 2
executed 1
```
which is what the spec dictates as the correct behavior and was incorrect in Babel 5.
2. Static class property initializers are evaluated once up front.
If you decorate a static class property, you will get a descriptor with an `initializer` property.
However whereas with Babel 5 this could be re-executed multiple times with potentially differing
results, `decorators-legacy` will precompute the value and return an initializer that will
return that value. e.g.
```
function dec(target, prop, descriptor){
let {initializer} = descriptor;
delete descriptor.initializer;
delete descriptor.writable;
descriptor.get = function(){
return initializer.call(this);
};
}
var i = 0;
class Example {
@dec
static prop = i++;
}
```
In Babel 5, every access to `prop` would increment `i`.
In Babel 6, the very first value of `i` will be cached for future `initializer` calls.
The spec is a little vague around how initializers work for repeat calls, and I'd consider
calling an `initializer` multiple times to be a mistake in general, so hopefully this will
not cause anyone trouble.
## License
MIT (c) 2015
|
