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@babel/plugin-proposal-decorators

Example​

Simple class decorator​

JavaScript
@annotation
class MyClass {}

function annotation(target) {
  target.annotated = true;
}

Class decorator​

JavaScript
@isTestable(true)
class MyClass {}

function isTestable(value) {
  return function decorator(target) {
    target.isTestable = value;
  };
}

Class method decorator​

JavaScript
class C {
  message = "hello!";

  @bound
  m() {
    console.log(this.message);
  }
}

function bound(value, { name, addInitializer }) {
  addInitializer(function () {
    this[name] = this[name].bind(this);
  });
}

Installation​

npm install --save-dev @babel/plugin-proposal-decorators

Usage​

babel.config.json
{
  "plugins": [
    ["@babel/plugin-proposal-decorators", { "version": "2023-11" }]
  ]
}

Via Node API​

JavaScript
require("@babel/core").transformSync("code", {
  plugins: [
    ["@babel/plugin-proposal-decorators", { version: "2023-11" }],
  ]
});

Options​

History
VersionChanges
v7.24.0Added support for version: "2023-11"
v7.22.0Added support for version: "2023-05"
v7.21.0Added support for version: "2023-01"
v7.19.0Added support for version: "2022-03"
v7.17.0Added the version option with support for "2021-12", "2018-09" and "legacy"

version​

"2023-11", "2023-05", "2023-01", "2022-03", "2021-12", "2018-09" or "legacy".

Selects the decorators proposal to use:

  • "2023-11" is the proposal version after the updates that reached consensus in the November 2023 TC30 meetings, intergrating this change
  • "2023-05" is the proposal version after the updates that reached consensus in the March and May 2023 TC39 meetings, integrating these changes.
  • "2023-01" is the proposal version after the updates that reached consensus in the January 2023 TC39 meeting, integrating pzuraq/ecma262#4.
  • "2022-03" is the proposal version that reached consensus for Stage 3 in the March 2022 TC39 meeting. You can read more about it at tc39/proposal-decorators@8ca65c046d.
  • "2021-12" is the proposal version as it was presented to TC39 in Dec 2021. You can read more about it at tc39/proposal-decorators@d6c056fa06.
  • "2018-09" is the proposal version that was initially promoted to Stage 2 presented to TC39 in Sept 2018. You can read more about it at tc39/proposal-decorators@7fa580b40f.
  • legacy is the legacy Stage 1 proposal, defined at wycats/javascript-decorators@e1bf8d41bf. The legacy mode will not have feature updates, and there are known discrepancies between Babel and TypeScript. It's recommended to migrate to the "2023-11" proposal.
caution

Babel 8 will only support "2023-11" and "legacy". If you are using a different decorators version, it's recommended to migrate to "2023-11".

The spec repo provides a brief summary of the differences between these versions.

If you specify the decoratorsBeforeExport option, version defaults to "2018-09", otherwise it is a required option.

decoratorsBeforeExport​

This option:

  • is disallowed when using version: "legacy", version: "2022-03", version: "2023-01", version: "2023-05" or version: "2023-11";
  • is required when using version: "2018-09";
  • is optional and defaults to false when using version: "2021-12".

boolean

JavaScript
// decoratorsBeforeExport: false
export @decorator class Bar {}

// decoratorsBeforeExport: true
@decorator
export class Foo {}

This option was originally added to help tc39 collect feedback from the community by allowing experimentation with the proposed syntaxes. The proposal has now settled on allowing decorators either before or after export.

legacy​

Deprecated

Use version: "legacy" instead. This option is a legacy alias.

boolean, defaults to false.

Use the legacy (stage 1) decorators syntax and behavior.

NOTE: Compatibility with @babel/plugin-transform-class-properties​

If you are including your plugins manually and using class elements transforms such as

  • @babel/plugin-transform-class-properties
  • @babel/plugin-transform-private-methods
  • @babel/plugin-transform-private-property-in-object
  • @babel/plugin-transform-class-static-block

make sure that @babel/plugin-proposal-decorators comes before them.

babel.config.json
{
  "plugins": [
-   "@babel/plugin-transform-class-properties",
    ["@babel/plugin-proposal-decorators", { "version": "2023-11" }]
+   "@babel/plugin-transform-class-properties"
  ]
}

If you are already using @babel/preset-env and Stage 3 decorators, you can safely remove the class elements transform, Babel will automatically apply decorators transform before any presets:

babel.config.json
{
  "presets": [
    ["@babel/preset-env"],
  ],
  "plugins": [
-   "@babel/plugin-transform-class-properties",
    ["@babel/plugin-proposal-decorators", { "version": "2023-11" }]
  ]
}

If you are using @babel/preset-env and legacy decorators, you must ensure the class elements transform is enabled regardless of your targets, because Babel only supports compiling legacy decorators when also compiling class properties:

babel.config.json
{
  "presets": [
    ["@babel/preset-env", {
+     "include": [
+       "@babel/plugin-transform-class-properties"
+     ]
    }],
  ],
  "plugins": [
-   "@babel/plugin-transform-class-properties",
    ["@babel/plugin-proposal-decorators", { "version": "legacy" }]
  ]
}

The include option will enable the transforms included in @babel/preset-env so you can safely remove them from your package.json.

tip

You can read more about configuring plugin options here

References​