Robust Instance Validation via Brand Checks

Snippet

Robust Instance Validation via Brand Checks

Traditional 'instanceof' checks can be unreliable if objects come from different execution contexts (realms). Private 'brand checks' using the '#field in object' syntax provide a foolproof way to verify that an object is a genuine instance of a class.

snippet.js
javascript
class Validator {
  #isValidator = true;
 
  static isValidator(obj) {
    return obj && #isValidator in obj;
  }
 
  validate(data) {
    return !!data;
  }
}
 
const v = new Validator();
console.log(Validator.isValidator(v)); // true
console.log(Validator.isValidator({ validate: () => {} })); // false

nodejs

Breakdown

#isValidator = true;

Declares a private field that acts as a unique identifier ('brand') for the class.

return obj && #isValidator in obj;

Uses the 'in' operator on a private field to perform a crash-proof brand check.

Validator.isValidator(...)

Provides a static utility to verify internal class integrity without exposing private data.

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Robust Instance Validation via Brand Checks

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