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112

How to override a defined (get-)property on a prototype?

rated 0 times [ 116] [ 4] / answers: 1 / hits: 29068 / 10 Years ago, fri, october 10, 2014, 12:00:00

I have some code which defines a getter (but no setter, if such is relevant) on a prototype. The value returned is correct in 99.99% of the cases; however, the goal is to set the property to evaluate to a different value for a specific object.

foo = {}

Object.defineProperty(foo, bar, {

    // only returns odd die sides

    get: function () { return (Math.random() * 6) | 1; }

});



x = Object.create(foo);

x.bar       // => eg. 5

x.bar = 4   // by fair dice roll

x.bar       // nope => eg. 3

How can the property be overridden for x, an existing object, such that it is assignable (eg. has default property behavior)?

Addendum: While a new property (value or get/set) can defined on x, I am looking for if there is a way to stop the behavior of a property in the [prototype] and turn bar back into a normal/ad-hoc property for the specific instance.

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mary

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2 Years ago

answered 10 Years ago monicag · Accepted Answer

By using Object.defineProperty on x:

var foo = {}

Object.defineProperty(foo, bar, {

    // only returns odd die sides

    get: function () { return (Math.random() * 6) | 1; }

});



var x = Object.create(foo);

display(x.bar);      // E.g. 5



(function() {

  var bar;

  var proto = Object.getPrototypeOf(x); // Or just use foo



  Object.defineProperty(x, bar, {

    get: function () { return typeof bar !== undefined ? bar : proto.bar; },

    set: function(value) { bar = value; }

  });

})();



display(x.bar);  // Still odd

x.bar = 4;       // By fair dice roll

display(x.bar);  // Shows 4



function display(msg) {

  document.body.insertAdjacentHTML(beforeend, <p> + msg + </p>);

}

I am looking for if there is a way to stop the behavior of a property in the [prototype] and turn bar back into a normal/ad-hoc property.

Okay, that's slightly different, but still uses Object.defineProperty:

var foo = {}

Object.defineProperty(foo, bar, {

    // only returns odd die sides

    get: function () { return (Math.random() * 6) | 1; }

});



var x = Object.create(foo);

display(x.bar);      // E.g. 5



Object.defineProperty(x, bar, {

  value: undefined,

  writable: true,

  enumerable: true // Or leave off if you want it non-enumerable

});



display(x.bar);  // undefined

x.bar = 4;       // By fair dice roll

display(x.bar);  // Shows 4



function display(msg) {

  document.body.insertAdjacentHTML(beforeend, <p> + msg + </p>);

}