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@babel/plugin-transform-private-property-in-object

NOTE: This plugin is included in @babel/preset-env, in ES2022

Example

In

JavaScript
class Foo {
#bar = "bar";

test(obj) {
return #bar in obj;
}
}

Out

JavaScript
class Foo {
constructor() {
_bar.set(this, {
writable: true,
value: "bar",
});
}

test() {
return _bar.has(this);
}
}

var _bar = new WeakMap();

Installation

npm install --save-dev @babel/plugin-transform-private-property-in-object

Usage

babel.config.json
{
"plugins": ["@babel/plugin-transform-private-property-in-object"]
}

Via CLI

Shell
babel --plugins @babel/plugin-transform-private-property-in-object

Via Node API

JavaScript
require("@babel/core").transformSync("code", {
plugins: ["@babel/plugin-transform-private-property-in-object"],
});

Options

loose

boolean, defaults to false.

Note: The loose mode configuration setting must be the same as @babel/transform-class-properties.

When true, private property in expressions will check own properties (as opposed to inherited ones) on the object, instead of checking for presence inside a WeakSet. This results in improved performance and debugging (normal property access vs .get()) at the expense of potentially leaking "privates" via things like Object.getOwnPropertyNames.

⚠️ Consider migrating to the top level privateFieldsAsProperties assumption.

babel.config.json
{
"assumptions": {
"privateFieldsAsProperties": true,
"setPublicClassFields": true
}
}

Note that both privateFieldsAsProperties and setPublicClassFields must be set to true.

Example

In

JavaScript
class Foo {
#bar = "bar";

test(obj) {
return #bar in obj;
}
}

Out

JavaScript
class Foo {
constructor() {
Object.defineProperty(this, _bar, {
writable: true,
value: "bar",
});
}

test() {
return Object.prototype.hasOwnProperty.call(this, _bar);
}
}

var _bar = babelHelpers.classPrivateFieldLooseKey("bar");

You can read more about configuring plugin options here

References