JavaScript Object

constructor

Object is the base in the prototype chain.

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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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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Constructors

new Object() : Object

Same as Object().

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new Object(value : Object) : Object

Same as Object(value).

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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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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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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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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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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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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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Object Methods

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

Returns a new Object with prototype equal to prototype. Each property on propertyDescriptors will be added to the new object.

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

For each property on propertyDescriptors, defines a new property on obj with the same name and supplied descripton. Returns obj.

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

Defines a new property with name equal to propertyName on obj with the supplied description. Returns obj.

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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freeze(obj : Object) : Object

For each property on obj, sets its property description's configurable property to false and 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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getOwnPropertyDescriptor(obj : Object, propertyName : String) : Object

Returns an Object that describes the propertyName property on obj.

Example:

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

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getOwnPropertyNames(obj : Object) : Array

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

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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getPrototypeOf(obj : Object) : Object

Returns the prototype of obj.

Example:

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

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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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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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keys(obj : Object) : Array

Returns an Array containing the enumerable properties on obj.

Example:

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

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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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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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