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  1. JDK
  2. JDK-8023368

Instance __proto__ property should exist and be writable.



    • Enhancement
    • Status: Resolved
    • P2
    • Resolution: Fixed
    • 8
    • 8
    • core-libs
    • None
    • b106
    • generic
    • generic


      Although having a mutable __proto__ property doesn't make for a consensus (https://bugzilla.mozilla.org/show_bug.cgi?id=642500) the analysis of node, npm and top 20 most depended upon modules (plus associated dependencies and testing frameworks) shows that the writable __proto__ property is used by JS developers.

      The following 5 patterns are quite common and not supported by nashorn.

      1) CRITICAL: Update prototype of an Object instance or a function:
       - function updateProto(a) { a.__proto__ = aNewProto; }

      2) MAJOR: Class inheritency :
       - DaughterClass.prototype.__proto__ = MotherClass.prototype;

      3) MAJOR: Provide a prototype to an Object instance at creation time:
        - var a = { __proto__: aProto, myprop: 9, … };
        - var a = new A(); a.__proto__ = aProto;

      4) MINOR : Access to __proto__ reference:
       - x.__proto__ === y.__proto__
       - x.__proto__.constructor

      5) MINOR: Provide a prototype to a Function at creation time:
       - var f = function() {}; f.__proto__ = aNewProto;

      Usage in most depended upon node modules
      1 occurs in 4 modules: proto-list, express, qs, ejs modules
      2 occurs in 15 modules: socket-io, commander, connect, connect-redis, engine-io, send, stylus, browser-builtins, supertest, superagent, emitter-component, mocha, jade, node-unit, cli-table modules
      3 occurs in 4 modules: traverse, hashish, express, esprima module test dependency modules
      4 occurs in 4 modules: traverse module test script, readable-stream module test script, jade, insert-module-globals modules
      5 occurs in 1 module: engine.io

      Workaround and side effects
      1 can't be workarounded. Script needs to be rewritten in order to propagate prototype updates.

      2 can be workarounded with Object.create but DaughterClass prototype can't be updated once Object.create has been called. Furthermore, any instance created prior to the extension are not updated.
      DaughterClass.prototype = Object.create(MotherClass.prototype);
      DaughterClass.prototype.constructor = DaughterClass;

      3 can be workarounded with Object.create
      Object.create(aProto, { myprop : {enumerable: true, value:9, ...} }

      4 can be workarounded with Object.getPrototypeOf
      Object.getPrototypeOf(x) === Object.getPrototypeOf(y)

      5 can be workarounded with:
      for(k in aNewProto) {
          f[k] = aNewProto[k];


        Issue Links



              sundar Sundararajan Athijegannathan
              jfdenise Jean-Francois Denise (Inactive)
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