JavaScript Object

constructor

Object is the base in the prototype chain. JavaScript Objects can have any number values stored as properties (specified by String names or Symbols).

Spec

Object() : Object

Same as new Object().

Spec

Object(value : Object) : Object

Same as new Object(value).

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Constructors

new Object() : Object

Creates an empty object. Can also be constructed as {}.

Example:

// The following are all equivalent
var x = {};
var y = Object();
var z = new Object();

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Spec

new Object(value : Object) : Object

If value is a primitive type, creates a box for value. Otherwise, returns value.

Example:

var x = true;
console.log(typeof x + ' / ' + typeof Object(x));
console.log(x === Object(x));
var y = {};
console.log(typeof y + ' / ' + typeof Object(y));
console.log(y === Object(y));

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Spec

Instance Indexers

this[propertyName : String] : Object

Returns the value stored in this at the specified propertyName. This is the same as the . style access but also allows the property name to be specified at runtime using a String of being hardcoded and allows access to properties that are invalid identifiers (such as including spaces or starting with a number).

Example:

var x = { foo: 'bar'};

// The following are equivalent
console.log(x.foo);
console.log(x['foo']);

// Use [] indexing when the name is not known ahead of time
var propertyName = 'f' + 'oo';
console.log(x[propertyName]);

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this[symbol : Symbol] : Object

Returns the value stored in this at the specified symbol.

Example:

var x = { foo: 'bar'};

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ECMAScript 2015

Instance Methods

constructor : Function

The Function that constructed this.

Example:

console.log({}.constructor === Object);
console.log([].constructor === Array);
console.log('foo'.constructor === String);

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hasOwnProperty(propertyName : String) : Boolean

Returns true if this has a property named propertyName stored on it. This method can be used to see if a property is set on the object, even if the value is set to undefined. Properties stored in the prototype chain do not count.

Example:

var x = { foo: 1 };
console.log(x.hasOwnProperty('foo'));
console.log(x.hasOwnProperty('bar'));

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Spec

isPrototypeOf(obj : Object) : Boolean

Returns true if this is in the prototype chain of obj.

Example:

// Array instances are Objects
console.log(Object.prototype.isPrototypeOf(Array.prototype));
// Array instances are not Functions
console.log(Function.prototype.isPrototypeOf(Array.prototype));
// Array constructor is a Function
console.log(Function.prototype.isPrototypeOf(Array));

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Spec

propertyIsEnumerable(propertyName : String) : Boolean

Returns true if the property named propertyName on this is enumerable and should be returned during a for (x in y) loop.

Example:

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

for (var propertyName in x) {
  console.log(propertyName + ' is ' + x[propertyName]);
}

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Spec

toLocaleString() : String

Returns a locale specific (if possible) string representation of this.

Example:

var x = 1.234;
console.log(x.toString());
console.log(x.toLocaleString());

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Spec

toString() : String

Returns a string representation of this. This method is called each time the object is used in a place a string is expected. Define your own toString to give a better string representation to your objects.

Example:

var pt = { x: 1, y: 2 };
console.log(pt);
pt.toString = function() { 
  return '(' + this.x + ', ' + this.y + ')';
};
console.log(pt);

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Spec

valueOf() : Object

For boxed primitive types, returns the unboxed value.

Example:

var x = Object(1);
console.log(x === 1);
console.log(x.valueOf() === 1);

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Spec

Object Methods

create(prototype : Object, [propertyDescriptors : Object]) : Object

Returns a new Object with prototype equal to prototype. Pass null for prototype to create an object with no prototype. Calls defineProperties(obj, propertyDescriptors) with the new object if propertyDescriptors is specified.

Example:

var x = Object.create({}, { foo: { value: 1, 
                                   writable: true,
                                   enumerable: true,
                                   configurable: true },
                            bar: { value: 2, 
                                   writable: true,
                                   enumerable: false,
                                   configurable: true }});
console.log(x.foo);
console.log(x.bar);
console.log(Object.keys(x));
console.log(Object.getOwnPropertyNames(x));

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Spec

defineProperties(obj : Object, propertyDescriptors : Object) : Object

For each property on propertyDescriptors, defines a new property on obj with the same name and supplied PropertyDescriptor. Returns obj. See also getOwnPropertyDescriptors().

Example:

var x = {};
Object.defineProperties(x, { foo: { value: 1, 
                                    writable: true,
                                    enumerable: true,
                                    configurable: true },
                             bar: { value: 2, 
                                    writable: true,
                                    enumerable: false,
                                    configurable: true }});
console.log(x.foo);
console.log(x.bar);
console.log(Object.keys(x));
console.log(Object.getOwnPropertyNames(x));

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Spec

defineProperty(obj : Object, propertyName : String, propertyDescriptor : PropertyDescriptor) : Object

Defines a new property with name equal to propertyName on obj with the supplied descriptor. Returns obj. See Reflect.defineProperty().

Example:

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

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Spec

entries(obj : Object) : Array<Array>

Returns an Array of key/value pairs for the properies of obj. See also fromEntries(), keys(), and values().

Example:

var x = { foo: 1, bar: 2 };
console.dir(Object.entries(x));

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ECMAScript 2017 Spec

freeze(obj : Object) : Object

For each property on obj, sets its PropertyDescriptor.configurable property to false and PropertyDescriptor.writable property to false. Also calls preventExtensions on obj. Returns obj.

Example:

var x = { foo: 1 };
Object.freeze(x);
x.foo = 3;
console.log(x.foo);
x.bar = 1;
console.log(x.bar);

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Spec

fromEntries(keyValuePairs : Iterator<Array>) : Object

Constructs a new Object from the specified key-value pairs. See also entries().

Example:

var x = [['foo', 1], ['bar', 2]];
console.dir(Object.fromEntries(x));

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ECMAScript 2019 Spec

getOwnPropertyDescriptor(obj : Object, propertyName : String) : PropertyDescriptor

Returns a PropertyDescriptor that describes the propertyName property on obj.

Example:

var x = { foo: 1,
          get bar() {
            return 'a';
          } };
Object.defineProperty(x, 'baz', { value: 2,
                                  writable: true,
                                  enumerable: false,
                                  configurable: true } );
console.dir(Object.getOwnPropertyDescriptor(x, 'foo'));
console.dir(Object.getOwnPropertyDescriptor(x, 'bar'));
console.dir(Object.getOwnPropertyDescriptor(x, 'baz'));

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Spec

getOwnPropertyDescriptors(obj : Object) : PropertyDescriptor

Returns an object key-value pairs of the property name and PropertyDescriptors that describes obj. See also getOwnPropertyDescriptor and create() and defineProperties().

Example:

var x = { foo: 1,
          get bar() {
            return 'a';
          } };
Object.defineProperty(x, 'baz', { value: 2,
                                  writable: true,
                                  enumerable: false,
                                  configurable: true } );
var descriptors = Object.getOwnPropertyDescriptors(x);
for (var p in descriptors) {
  console.log(p + ':');
  console.dir(descriptors[p]);
  console.log();
}

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Spec

getOwnPropertyNames(obj : Object) : Array<String>

Returns an Array of the names of all properties set on obj including those that are not enumerable. See also keys().

Example:

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

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Spec

getOwnPropertySymbols(obj : Object) : Array<Symbol>

Returns an Array of the symbols of properties set on obj including those that are not enumerable.

Example:

var myObject = { foo: 'bar' };

var mySymbol = Symbol('MySymbol');

myObject[mySymbol] = 'baz';

for (var symbol of Object.getOwnPropertySymbols(myObject)) {
  console.log('myObject[' + symbol.toString() + '] = ' + myObject[symbol]);
}

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ECMAScript 2015 Spec

getPrototypeOf(obj : Object) : Object

Returns the prototype of obj.

Example:

console.log(Object.getPrototypeOf([]) === Array.prototype);

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Spec

is(obj1 : Object, obj2 : Object) : Boolean

Returns true if obj1 is identical to obj2. This is mostly the same as obj1 === obj2 except for some Number comparisons involving NaN, 0, and -0.

Example:

console.log(0 === -0);
console.log(Object.is(0, -0));
console.log();

console.log(NaN === NaN);
console.log(Object.is(NaN, NaN));

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Spec

isExtensible(obj : Object) : Boolean

Returns false if preventExtensions was called on obj. A non-extensible object cannot have new properties added to it.

Example:

var x = { foo: 1 };
console.log(Object.isExtensible(x));
Object.preventExtensions(x);
console.log(Object.isExtensible(x));
console.log(Object.isSealed(x));
console.log(Object.isFrozen(x));

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Spec

isFrozen(obj : Object) : Boolean

Returns true if freeze was called on obj. A frozen object cannot have new properties added and existing properties cannot be modified.

Example:

var x = { foo: 1 };
Object.freeze(x);
console.log(Object.isSealed(x));
console.log(Object.isExtensible(x));
console.log(Object.isFrozen(x));

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isSealed(obj : Object) : Boolean

Returns true if seal was called on obj. A sealed object cannot have new properties added to it but the value of existing properties can be changed.

Example:

var x = { foo: 1 };
Object.seal(x);
console.log(Object.isSealed(x));
console.log(Object.isExtensible(x));
console.log(Object.isFrozen(x));

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Spec

keys(obj : Object) : Array<String>

Returns an Array containing the enumerable properties on obj. See also getOwnPropertyNames(), entries() and values().

Example:

var x = { foo: 1, bar: 2 };
console.log(Object.keys(x));

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Spec

preventExtensions(obj : Object) : Object

Prevents new properties from being added to obj. Unlike freeze, existing properties can be modified. Returns obj.

Example:

var x = { foo: 1 };
Object.preventExtensions(x);
x.foo = 3;
console.log(x.foo);
x.bar = 1;
console.log(x.bar);

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Spec

seal(obj : Object) : Object

Same as preventExtensions except it also sets each property as not configurable. Returns obj.

Example:

var x = { foo: 1 };
Object.seal(x);
x.foo = 3;
console.log(x.foo);
x.bar = 1;
console.log(x.bar);

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Spec

values(obj : Object) : Array<Object>

Returns an Array containing the enumerable property values on obj. See also entries() and keys().

Example:

var x = { foo: 1, bar: 2 };
console.log(Object.values(x));

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ECMAScript 2017 Spec

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