Syntax error - Support for the experimental syntax 'decorators-legacy' isn't currently enabled

Question

I'm trying to build JS react project with decorators. My .babelrc looks like this:

{
  "presets": [
    "@babel/preset-env",
    "@babel/preset-react",

  ],
  "plugins": [
    "@babel/plugin-transform-runtime",
    "@babel/plugin-transform-object-assign",
    [
      "@babel/plugin-proposal-decorators",
      {
        "legacy": true
      }
    ],
    ["@babel/plugin-proposal-class-properties", { "loose": true }]
  ]
}

Adding @babel/plugin-proposal-decorators problems appears again.

I am using babel 7, webpack 4 and react 16.5

webpack.config.js:

const path = require("path");
const webpack = require("webpack");
const componentName = "reports-desktop";
const publicFolderRelativePath = "../../../../../../public/js";
const ignorePlugin = new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/);

module.exports = {
    entry: './reports-desktop.js'
    ,
    output: {
        path: path.resolve(__dirname, publicFolderRelativePath),
        filename: `${componentName}.js`
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                exclude: /node_modules/,
                use: {
                    loader: "babel-loader"
                }
            }
        ]
    },
    plugins: [
        ignorePlugin
    ]
};

package.json:

{
  "name": "captain",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "watch": "webpack -w --mode development --progress --color --display-error-details",
    "build": "webpack --mode production"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/core": "^7.0.0",
    "@babel/plugin-proposal-class-properties": "^7.0.0",
    "@babel/plugin-proposal-decorators": "^7.0.0",
    "@babel/plugin-transform-object-assign": "^7.0.0",
    "@babel/plugin-transform-runtime": "^7.0.0",
    "@babel/preset-env": "^7.0.0",
    "@babel/preset-react": "^7.0.0",
    "@babel/preset-stage-1": "^7.0.0",
    "@babel/preset-stage-2": "^7.0.0",
    "babel-loader": "^8.0.2",
    "babel-plugin-transform-decorators": "^6.24.1",
    "react": "^16.5.0",
    "react-dom": "^16.5.0",
    "react-redux": "^5.0.7",
    "react-router-dom": "^4.3.1",
    "redux": "^4.0.0",
    "webpack": "^4.17.3",
    "webpack-cli": "^3.1.0"
  },
  "dependencies": {
    "axios": "^0.18.0",
    "dropzone": "^5.5.1",
    "lodash": "^4.17.10",
    "moment": "^2.22.2",
    "prop-types": "^15.6.2",
    "react-addons-update": "^15.6.2",
    "react-bootstrap": "^0.32.4",
    "react-datetime": "^2.15.0",
    "react-dnd": "^5.0.0",
    "react-dnd-html5-backend": "^5.0.1",
    "react-media": "^1.8.0",
    "react-tooltip": "^3.8.1"
  }
}

Am I maybe using @babel/plugin-proposal-decorators wrong? As it says in documentation this should fix my problem, but it still appears.

Answer 1

I had the same problem, but I was able to get it working by running npm install --save-dev @babel/plugin-proposal-decorators and adding ["@babel/plugin-proposal-decorators", { "legacy": true }] to the plugins section in my .babelrc.

The plugins section of .babelrc, for me, now looks like this:

"plugins": [
  ["@babel/plugin-proposal-decorators", { "legacy": true }]
]

Answer 2

First, execute the command:

npm install customize-cra react-app-rewired @babel/plugin-proposal-decorators --save

Create a new file config-overrides.js at project root and then execute the following:

const { override, addDecoratorsLegacy } = require('customize-cra');
module.exports = override(
 addDecoratorsLegacy()
 );

Also, edit your package.jsonfile ：

"scripts": {
 "start": "react-app-rewired start",
 "build": "react-app-rewired build",
 "test": "react-app-rewired test",
 "eject": "react-app-rewired eject"
  },

then restart.

Answer 3

Taken from mobxjs. If you're still having issues refer to this. It helped me.

Example config for babel 7, in decorators legacy mode:

//.babelrc

{
    "presets": ["@babel/preset-env"],
    "plugins": [
        ["@babel/plugin-proposal-decorators", { "legacy": true }],
        ["@babel/plugin-proposal-class-properties", { "loose": true }]
    ]
}

Please note that plugin ordering is important, and plugin-proposal-decorators should be the first plugin in your plugin list

"devDependencies": {
    "@babel/core": "^7.1.0",
    "@babel/plugin-proposal-class-properties": "^7.1.0",
    "@babel/plugin-proposal-decorators": "^7.1.0",
    "@babel/preset-env": "^7.1.0"
}

Non-legacy mode decorators (stage 2) is work in progress, see #1732

Edit: updated config to show the non-beta configuration for babel 7

Answer 4

I fixed this with yarn add @babel/plugin-proposal-decorators

Then I added the following to babel.config.js in the plugins section:

  [
    require('@babel/plugin-proposal-decorators').default,
    {
      legacy: true
    }
  ],

Finally I needed to restart my webpack dev server.

---

I haven't tested this but like @christopher bradshaw answers says and according to the babel website if you are using .babelrc for configuration then add the following to the "plugins" section:

  ["@babel/plugin-proposal-decorators", { "legacy": true }]

Answer 5

If you face this error while using ReactJS with MobX, don't enable decorator syntax, but leverage the MobX's built-in decorate utility to apply decorators to your classes / objects.

Don't:

import { observable, computed, action } from "mobx";

class Timer {
  @observable start = Date.now();
  @observable current = Date.now();

  @computed
  get elapsedTime() {
    return this.current - this.start + "milliseconds";
  }

  @action
  tick() {
    this.current = Date.now();
  }
}

Do:

import { observable, computed, action, decorate } from "mobx";

class Timer {
  start = Date.now();
  current = Date.now();

  get elapsedTime() {
    return this.current - this.start + "milliseconds";
  }

  tick() {
    this.current = Date.now();
  }
}
decorate(Timer, {
  start: observable,
  current: observable,
  elapsedTime: computed,
  tick: action
});