1.3.5 • Public • Published

    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

      "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.

    /// WRONG
    "plugins": [
    // RIGHT
    "plugins": [

    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 {

      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(){
      var i = 0;
      class Example {
          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.


    MIT (c) 2015


    npm i babel-plugin-transform-decorators-legacy

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