

@babel/plugin-proposal-private-property-in-object NOTE: This plugin is included in @babel/preset-env, in ES2022  
Example In class Foo {
  #bar = "bar";

  test(obj) {
    return #bar in obj;
  }
}
 Out 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-proposal-private-property-in-object
 
Usage 
With a configuration file (Recommended) {
  "plugins": ["@babel/plugin-proposal-private-property-in-object"]
}
 
Via CLI babel --plugins @babel/plugin-proposal-private-property-in-object
 
Via Node API require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-proposal-private-property-in-object"],
});
 
Options 
loose
 boolean, defaults to false.  Note: The loose mode configuration setting must be the same as @babel/proposal-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 class Foo {
  #bar = "bar";

  test(obj) {
    return #bar in obj;
  }
}
 Out 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  Proposal: Ergonomic brand checks for Private Fields  



    
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