JavaScript PropertyDescriptor : Object

A PropertyDescriptor describes a property on an Object. Any JavaScript object can be used as a PropertyDescriptor where unspecified properties will be treated as undefined or false.

Spec

Instance Properties

configurable : Boolean

Set to true if the property descriptor can be changed later. See the Object.freeze() and Object.seal() methods for ways to set configurable to false on all properties on an object.

Example:

var x = {};
Object.defineProperty(x, 'foo', { value: 1, 
                                  writable: false,
                                  enumerable: true,
                                  configurable: false } );

try {
  // Attempt to override non-configurable property descriptor
  Object.defineProperty(x, 'foo', { value: 1, 
                                    writable: true,
                                    enumerable: true,
                                    configurable: false } );
} catch(e) {
  console.log('Error:');
  console.log(e);
}

Run

Results:

enumerable : Boolean

Set to true if the property is accessed during for (propertyName in object). See the Object.propertyIsEnumerable() method.

Example:

var x = { foo: 1 };
Object.defineProperty(x, 'bar', { value: 2, 
                                  writable: true,
                                  enumerable: false,
                                  configurable: true } );

// 'foo' is enumerable. 'bar' in not enumerable
for (var propertyName in x) {
  console.log(propertyName + ' is ' + x[propertyName]);
}

Run

Results:

get : getFunction

getFunction() : Object

The getter function that returns the value for the property. Use set to specified the setter function. If get or set functions are specified, value and writable cannot be specified.

Example:

var x = { get foo() {
            console.log('in x.foo getter');
            return this._foo;
          },
          set foo(value) {
            console.log('in x.foo setter. value = ' + value);
            this._foo = value;
          }
        };

console.log('setting x.foo to 1');
x.foo = 1;

console.log('getting x.foo');
console.log(x.foo);

// Equivalent definition of 'foo' using a PropertyDescriptor
var y = {};
Object.defineProperty(y, 'foo', { get: function() {
                                    console.log('in y.foo getter');
                                    return this._foo;
                                  },
                                  set: function() {
                                    console.log('in y.foo setter. value = ' + value);
                                    return this._foo;
                                  },
                                  enumerable: true,
                                  configurable: true } );

Run

Results:

set : setFunction

setFunction(value : Object) : undefined

The setter function that is called when setting the property. Use get to specified the getter function. If get or set functions are specified, value and writable cannot be specified.

Example:

var x = { get foo() {
            console.log('in x.foo getter');
            return this._foo;
          },
          set foo(value) {
            console.log('in x.foo setter. value = ' + value);
            this._foo = value;
          }
        };

console.log('setting x.foo to 1');
x.foo = 1;

console.log('getting x.foo');
console.log(x.foo);

// Equivalent definition of 'foo' using a PropertyDescriptor
var y = {};
Object.defineProperty(y, 'foo', { get: function() {
                                    console.log('in y.foo getter');
                                    return this._foo;
                                  },
                                  set: function() {
                                    console.log('in y.foo setter. value = ' + value);
                                    return this._foo;
                                  },
                                  enumerable: true,
                                  configurable: true } );

Run

Results:

value : Object

The value stored in the property. value can only be specified if both get and set functions are not specified.

Example:

var x = { foo: 1 };
Object.defineProperty(x, 'bar', { value: 2, enumerable: true } );
for (var propertyName in x) {
  console.log(propertyName + ' value is ' + x[propertyName]);
}

Run

Results:

writable : Boolean

Set to true if the property can be modified, false otherwise. writable can only be specified if both get and set functions are not specified.

Example:

var x = { foo: 1 };
Object.defineProperty(x, 'bar', { value: 2,
                                  writable: false } );

x.foo = 1234;
console.log(x.foo);

// bar is not writable.  Setting it silently fails.
x.bar = 1234;
console.log(x.bar);

Run

Results:

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