Is it possible to implement dynamic getters/setters in JavaScript?

I am aware of how to create getters and setters for properties whose names one already knows, by doing something like this:

// A trivial example:
function MyObject(val){
    this.count = 0;
    this.value = val;
}
MyObject.prototype = {
    get value(){
        return this.count < 2 ? "Go away" : this._value;
    },
    set value(val){
        this._value = val + (++this.count);
    }
};
var a = new MyObject('foo');

alert(a.value); // --> "Go away"
a.value = 'bar';
alert(a.value); // --> "bar2"

Now, my question is, is it possible to define sort of catch-all getters and setters like these? I.e., create getters and setters for any property name which isn't already defined.

The concept is possible in PHP using the __get() and __set() magic methods (see the PHP documentation for information on these), so I'm really asking is there a JavaScript equivalent to these?

Needless to say, I'd ideally like a solution that is cross-browser compatible.

2013 and 2015 Update (see below for the original answer from 2011):

This changed as of the ES2015 (aka "ES6") specification: JavaScript now has proxies. Proxies let you create objects that are true proxies for (facades on) other objects. Here's a simple example that turns any property values that are strings to all caps on retrieval:

"use strict";
if (typeof Proxy == "undefined") {
    throw new Error("This browser doesn't support Proxy");
}
let original = {
    "foo": "bar"
};
let proxy = new Proxy(original, {
    get(target, name, receiver) {
        let rv = Reflect.get(target, name, receiver);
        if (typeof rv === "string") {
            rv = rv.toUpperCase();
        }
        return rv;
      }
});
console.log(`original.foo = ${original.foo}`); // "original.foo = bar"
console.log(`proxy.foo = ${proxy.foo}`);       // "proxy.foo = BAR"

Operations you don't override have their default behavior. In the above, all we override is get, but there's a whole list of operations you can hook into.

In the get handler function's arguments list:

target is the object being proxied (original, in our case).
name is (of course) the name of the property being retrieved, which is usually a string but could also be a Symbol.
receiver is the object that should be used as this in the getter function if the property is an accessor rather than a data property. In the normal case this is the proxy or something that inherits from it, but it can be anything since the trap may be triggered by Reflect.get.

This lets you create an object with the catch-all getter and setter feature you want:

"use strict";
if (typeof Proxy == "undefined") {
    throw new Error("This browser doesn't support Proxy");
}
let obj = new Proxy({}, {
    get(target, name, receiver) {
        if (!Reflect.has(target, name)) {
            console.log("Getting non-existent property '" + name + "'");
            return undefined;
        }
        return Reflect.get(target, name, receiver);
    },
    set(target, name, value, receiver) {
        if (!Reflect.has(target, name)) {
            console.log(`Setting non-existent property '${name}', initial value: ${value}`);
        }
        return Reflect.set(target, name, value, receiver);
    }
});

console.log(`[before] obj.foo = ${obj.foo}`);
obj.foo = "bar";
console.log(`[after] obj.foo = ${obj.foo}`);

The output of the above is:

Getting non-existent property 'foo'
[before] obj.foo = undefined
Setting non-existent property 'foo', initial value: bar
[after] obj.foo = bar

Note how we get the "non-existent" message when we try to retrieve foo when it doesn't yet exist, and again when we create it, but not after that.

Answer from 2011 (see above for 2013 and 2015 updates):

No, JavaScript doesn't have a catch-all property feature. The accessor syntax you're using is covered in Section 11.1.5 of the spec, and doesn't offer any wildcard or something like that.

You could, of course, implement a function to do it, but I'm guessing you probably don't want to use f = obj.prop("foo"); rather than f = obj.foo; and obj.prop("foo", value); rather than obj.foo = value; (which would be necessary for the function to handle unknown properties).

FWIW, the getter function (I didn't bother with setter logic) would look something like this:

MyObject.prototype.prop = function(propName) {
    if (propName in this) {
        // This object or its prototype already has this property,
        // return the existing value.
        return this[propName];
    }

    // ...Catch-all, deal with undefined property here...
};

But again, I can't imagine you'd really want to do that, because of how it changes how you use the object.

The following could be an original approach to this problem:

var obj = {
  emptyValue: null,
  get: function(prop){
    if(typeof this[prop] == "undefined")
        return this.emptyValue;
    else
        return this[prop];
  },
  set: function(prop,value){
    this[prop] = value;
  }
}

In order to use it the properties should be passed as strings. So here is an example of how it works:

//To set a property
obj.set('myProperty','myValue');

//To get a property
var myVar = obj.get('myProperty');

Edit: An improved, more object-oriented approach based on what I proposed is the following:

function MyObject() {
    var emptyValue = null;
    var obj = {};
    this.get = function(prop){
        return (typeof obj[prop] == "undefined") ? emptyValue : obj[prop];
    };
    this.set = function(prop,value){
        obj[prop] = value;
    };
}

var newObj = new MyObject();
newObj.set('myProperty','MyValue');
alert(newObj.get('myProperty'));

You can see it working here.

Preface:

T.J. Crowder's answer mentions a Proxy, which will be needed for a catch-all getter/setter for properties which don't exist, as the OP was asking for. Depending on what behavior is actually wanted with dynamic getters/setters, a Proxy may not actually be necessary though; or, potentially, you may want to use a combination of a Proxy with what I'll show you below.

(P.S. I have experimented with Proxy thoroughly in Firefox on Linux recently and have found it to be very capable, but also somewhat confusing/difficult to work with and get right. More importantly, I have also found it to be quite slow (at least in relation to how optimized JavaScript tends to be nowadays) - I'm talking in the realm of deca-multiples slower.)

To implement dynamically created getters and setters specifically, you can use Object.defineProperty() or Object.defineProperties(). This is also quite fast.

The gist is that you can define a getter and/or setter on an object like so:

let obj = {};
let val = 0;
Object.defineProperty(obj, 'prop', { //<- This object is called a "property descriptor".
  //Alternatively, use: `get() {}`
  get: function() {
    return val;
  },
  //Alternatively, use: `set(newValue) {}`
  set: function(newValue) {
    val = newValue;
  }
});

//Calls the getter function.
console.log(obj.prop);
let copy = obj.prop;
//Etc.

//Calls the setter function.
obj.prop = 10;
++obj.prop;
//Etc.

Several things to note here:

You cannot use the value property in the property descriptor (not shown above) simultaneously with get and/or set; from the docs:

Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors. A data descriptor is a property that has a value, which may or may not be writable. An accessor descriptor is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.
Thus, you'll note that I created a val property outside of the Object.defineProperty() call/property descriptor. This is standard behavior.
As per the error here, don't set writable to true in the property descriptor if you use get or set.
You might want to consider setting configurable and enumerable, however, depending on what you're after; from the docs:
configurable
- true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.
- Defaults to false.
enumerable
- true if and only if this property shows up during enumeration of the properties on the corresponding object.
- Defaults to false.

On this note, these may also be of interest:

Object.getOwnPropertyNames(obj): gets all properties of an object, even non-enumerable ones (AFAIK this is the only way to do so!).
Object.getOwnPropertyDescriptor(obj, prop): gets the property descriptor of an object, the object that was passed to Object.defineProperty() above.
obj.propertyIsEnumerable(prop);: for an individual property on a specific object instance, call this function on the object instance to determine whether the specific property is enumerable or not.

I was looking for something and I figured out on my own.

/*
    This function takes an object and converts to a proxy object.
    It also takes care of proxying nested objectsa and array.
*/
let getProxy = (original) => {

    return new Proxy(original, {

        get(target, name, receiver) {
            let rv = Reflect.get(target, name, receiver);
            return rv;
        },

        set(target, name, value, receiver) {

            // Proxies new objects 
            if(typeof value === "object"){
                value = getProxy(value);
            }

            return Reflect.set(target, name, value, receiver);
        }
    })
}

let first = {};
let proxy = getProxy(first);

/*
    Here are the tests
*/

proxy.name={}                               // object
proxy.name.first={}                         // nested object
proxy.name.first.names=[]                   // nested array 
proxy.name.first.names[0]={first:"vetri"}   // nested array with an object

/*
    Here are the serialised values
*/
console.log(JSON.stringify(first))  // {"name":{"first":{"names":[{"first":"vetri"}]}}}
console.log(JSON.stringify(proxy))  // {"name":{"first":{"names":[{"first":"vetri"}]}}}

Is it possible to implement dynamic getters/setters in JavaScript?

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