override object accessot javascript

Question

I would like to override accessor of a javascript object so that instead of override it should return a fixed value.

eg.

var obj = {};
console.log(obj.someProperty) //will print undefined expected value false

Do you mean that, for any key that has no defined value, you would get back a specific constant? — JCOC611, Feb 06 '16 at 01:11
Yes. I want that instead of undefined it return a fixed constant for any undefined key. — Kushal Khandelwal, Feb 06 '16 at 01:13

Jonathan.Brink · Answer 1 · 2016-02-06T01:12:10.553

0

If a property is undefined there is no way to have it return false by default but it will be "falsey".

For example:

var obj = {};
if (!obj.someProperty) {
        console.log("expression is falsey");
}

A more explicit way to test truthiness is to use the double bang operator:

var obj = {};
console.log(!!obj.someProperty) // print's undefined, but evaluates to false

But in short, what you are going after will not work because in JavaScript you cannot redefine undefined.

edited Feb 06 '16 at 01:12

answered Feb 06 '16 at 01:10

Jonathan.Brink

Ya that will work if i know the name of property. But i want to do it for any property which is undefined. – Kushal Khandelwal Feb 06 '16 at 01:11

score 0 · Answer 2 · answered Feb 06 '16 at 01:15

I would do this with the following:

var obj = {};
if (!obj.hasOwnProperty('itdoesnthavethis')) console.log('property is not there')

This will always return a boolean and will not check through the object's prototype chain for the property, just the object itself.

score 0 · Answer 3 · answered Feb 06 '16 at 01:30

With ES6 and its Proxy feature you can do something like this:

var obj = {};

obj = new Proxy(obj, {
  get: function(target, prop) {
    var pv = target[prop];
    return pv === undefined ? false : pv;
  }
});

that will essentially create proxy wrapper around your object handling property get operations.

With ES5 something like that is impossible to achieve for an arbitrary property.

But if set of properties is known you can design your own object class:

function MyObject() {}
MyObject.prototype.foo = false;
MyObject.prototype.bar = false;
MyObject.prototype.baz = false;

and use it as:

var obj = new MyObject();

console.log(obj.foo); // false
console.log(obj.bar); // false
console.log(obj.foobar); // undefined

3 Answers3