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    What private fields, methods, or accessors does this class have?

    ⚠️ 🚨 ⚠️ DISCLAIMER This capability - observing the existence, name, and in many cases, contents of class private fields/methods/accessors - is not supposed to exist. The inability to in any way interact with private fields is a critical part of their design in the JavaScript language, and is the only thing that makes them truly "private".

    This is possible because node, in v12.5+ as of this writing, directly exposes the v8 engine's internal debugger protocol via the core inspector module. This should not be exposed to runtime code - it's meant for Chrome's devtools to be able to use, for debugging only. It will not work in any other environment.

    Please do not use this for any non-debugging purpose. Private fields, methods, and accessors, just like closed-over variables, variable names, or function argument names, are an internal implementation detail, and they can and likely will be changed in a non-semver-major update, because by definition they are not breaking changes - since they're not observable. Consider yourself warned.

    Note: this package will work down to node v8.3, but private fields support was added in node v12.0, and private methods/accessors were added in node v14.6.

    Getting started

    npm install --save private-fields


    import getPrivateFields from 'private-fields';
    import assert from 'assert';
    const objectValue = { a: 1 };
    const symbolValue = Symbol.iterator;
    class C {
    	#y = 3;
    	#foo() {}
    	get #z() {}
    	set #z(v) {}
    	#object = objectValue;
    	#symbol = symbolValue;
    const fields = await getPrivateFields(new C());
    assert.deepEqual(fields, [
    		name: '#foo',
    		type: 'function',
    		description: '#foo() {}', // functions are not provided, but their toString is
    		name: '#z',
    		get: { type: 'function', description: 'get #z() {}' },
    		set: { type: 'function', description: 'set #z(v) {}' },
    		name: '#x',
    		value: undefined,
    		name: '#y',
    		value: 3,
    		name: '#object',
    		clonedValue: { a: 1 },
    		name: '#symbol',
    		clonedValue: Symbol(Symbol.iterator),
    const [, , , , object, symbol] = fields;
    assert.notEqual(object, objectValue); // the original object is not provided
    assert.deepEqual(object, objectValue); // but its properties are cloned
    assert.notEqual(symbol.clonedValue, Symbol.iterator); // symbols are not provided
    assert.equal(symbol.description, Symbol.iterator.description); // but a symbol with the same description is


    Simply clone the repo, npm install, and run npm test


    npm i private-fields

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    • ljharb