Files
Creates an object wrapper.
The Object constructor creates an object wrapper for the given value. If the value is null or undefined, it will create and return an empty object, otherwise, it will return an object of a type that corresponds to the given value.
When called in a non-constructor context, Object behaves identically.
The following examples store an empty Object object in o: var o = new Object();
var o = new Object(undefined);
var o = new Object(null);
The following examples store Boolean objects in o:
// equivalent to o = new Boolean(true);
var o = new Object(true);
// equivalent to o = new Boolean(false);
var o = new Object(Boolean());
Specifies the function that creates an object's prototype.
Returns a reference to the Object function that created the instance's prototype. Note that the value of this property is a reference to the function itself, not a string containing the function's name, but it isn't read only (except for primitive Boolean, Number or String values: 1, true, "read-only").
All objects inherit a constructor property from their prototype:
o = new Object // or o = {} in JavaScript 1.2
o.constructor == Object
a = new Array // or a = [] in JavaScript 1.2
a.constructor == Array
n = new Number(3)
n.constructor == Number
Even though you cannot construct most HTML objects, you can do comparisons. For example,
document.constructor == Document
document.form3.constructor == Form
The following example creates a prototype, Tree, and an object of that type, theTree. The example then displays the constructor property for the object theTree.
function Tree(name) {
this.name = name;
}
theTree = new Tree("Redwood");
console.log("theTree.constructor is " + theTree.constructor);
This example displays the following output:
theTree.constructor is function Tree(name) {
this.name = name;
}
The following example shows how to modify constructor value of generic objects. Only true, 1 and "test" variable constructors will not be changed. This example explains that is not always so safe to believe in constructor function.
function Type(){};
var types = [
new Array, [],
new Boolean, true,
new Date,
new Error,
new Function, function(){},
Math,
new Number, 1,
new Object, {},
new RegExp, /(?:)/,
new String, "test"
];
for(var i = 0; i < types.length; i++){
types[i].constructor = Type;
types[i] = [types[i].constructor, types[i] instanceof Type, types[i].toString()];
};
alert(types.join("\n"));
Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.
Every object descended from Object inherits the hasOwnProperty method. This method can be used
to determine whether an object has the specified property as a direct property of that object;
unlike the in operator, this method does not check down the object's prototype chain.
The following example determines whether the o object contains a property named prop:
o = new Object();
o.prop = 'exists';
function changeO() {
o.newprop = o.prop;
delete o.prop;
}
o.hasOwnProperty('prop'); //returns true
changeO();
o.hasOwnProperty('prop'); //returns false
The following example differentiates between direct properties and properties inherited through the prototype chain:
o = new Object();
o.prop = 'exists';
o.hasOwnProperty('prop'); // returns true
o.hasOwnProperty('toString'); // returns false
o.hasOwnProperty('hasOwnProperty'); // returns false
The following example shows how to iterate over the properties of an object without executing on inherit properties.
var buz = {
fog: 'stack'
};
for (var name in buz) {
if (buz.hasOwnProperty(name)) {
alert("this is fog (" + name + ") for sure. Value: " + buz[name]);
}
else {
alert(name); // toString or something else
}
}
The name of the property to test.
Returns true if object contains specified property; else returns false.
Returns a boolean indication whether the specified object is in the prototype chain of the object this method is called upon.
isPrototypeOf allows you to check whether or not an object exists within another object's
prototype chain.
For example, consider the following prototype chain:
function Fee() {
// . . .
}
function Fi() {
// . . .
}
Fi.prototype = new Fee();
function Fo() {
// . . .
}
Fo.prototype = new Fi();
function Fum() {
// . . .
}
Fum.prototype = new Fo();
Later on down the road, if you instantiate Fum and need to check if Fi's prototype exists
within the Fum prototype chain, you could do this:
var fum = new Fum();
. . .
if (Fi.prototype.isPrototypeOf(fum)) {
// do something safe
}
This, along with the instanceof operator particularly comes in handy if you have code that can
only function when dealing with objects descended from a specific prototype chain, e.g., to
guarantee that certain methods or properties will be present on that object.
an object to be tested against each link in the prototype chain of the object argument
the object whose prototype chain will be searched
Returns true if object is a prototype and false if not.
Returns a boolean indicating if the internal ECMAScript DontEnum attribute is set.
Every object has a propertyIsEnumerable method. This method can determine whether the specified
property in an object can be enumerated by a for...in loop, with the exception of properties
inherited through the prototype chain. If the object does not have the specified property, this
method returns false.
The following example shows the use of propertyIsEnumerable on objects and arrays:
var o = {};
var a = [];
o.prop = 'is enumerable';
a[0] = 'is enumerable';
o.propertyIsEnumerable('prop'); // returns true
a.propertyIsEnumerable(0); // returns true
The following example demonstrates the enumerability of user-defined versus built-in properties:
var a = ['is enumerable'];
a.propertyIsEnumerable(0); // returns true
a.propertyIsEnumerable('length'); // returns false
Math.propertyIsEnumerable('random'); // returns false
this.propertyIsEnumerable('Math'); // returns false
Direct versus inherited properties
var a = [];
a.propertyIsEnumerable('constructor'); // returns false
function firstConstructor()
{
this.property = 'is not enumerable';
}
firstConstructor.prototype.firstMethod = function () {};
function secondConstructor()
{
this.method = function method() { return 'is enumerable'; };
}
secondConstructor.prototype = new firstConstructor;
secondConstructor.prototype.constructor = secondConstructor;
var o = new secondConstructor();
o.arbitraryProperty = 'is enumerable';
o.propertyIsEnumerable('arbitraryProperty'); // returns true
o.propertyIsEnumerable('method'); // returns true
o.propertyIsEnumerable('property'); // returns false
o.property = 'is enumerable';
o.propertyIsEnumerable('property'); // returns true
// These return false as they are on the prototype which
// propertyIsEnumerable does not consider (even though the last two
// are iteratable with for-in)
o.propertyIsEnumerable('prototype'); // returns false (as of JS 1.8.1/FF3.6)
o.propertyIsEnumerable('constructor'); // returns false
o.propertyIsEnumerable('firstMethod'); // returns false
The name of the property to test.
If the object does not have the specified property, this method returns false.
Returns a string representing the object. This method is meant to be overridden by derived objects for locale-specific purposes.
Object's toLocaleString returns the result of calling toString.
This function is provided to give objects a generic toLocaleString method, even though not all
may use it. Currently, only Array, Number, and Date override toLocaleString.
Object represented as a string.
Returns a string representation of the object.
Every object has a toString() method that is automatically called when the object is to be
represented as a text value or when an object is referred to in a manner in which a string is
expected. By default, the toString() method is inherited by every object descended from Object.
If this method is not overridden in a custom object, toString() returns "[object type]", where
type is the object type. The following code illustrates this:
var o = new Object();
o.toString(); // returns [object Object]
You can create a function to be called in place of the default toString() method. The
toString() method takes no arguments and should return a string. The toString() method you
create can be any value you want, but it will be most useful if it carries information about the
object.
The following code defines the Dog object type and creates theDog, an object of type Dog:
function Dog(name,breed,color,sex) {
this.name=name;
this.breed=breed;
this.color=color;
this.sex=sex;
}
theDog = new Dog("Gabby","Lab","chocolate","female");
If you call the toString() method on this custom object, it returns the default value inherited
from Object:
theDog.toString(); //returns [object Object]
The following code creates and assigns dogToString() to override the default toString() method.
This function generates a string containing the name, breed, color, and sex of the object, in the
form "property = value;".
Dog.prototype.toString = function dogToString() {
var ret = "Dog " + this.name + " is a " + this.sex + " " + this.color + " " + this.breed;
return ret;
}
With the preceding code in place, any time theDog is used in a string context, JavaScript
automatically calls the dogToString() function, which returns the following string:
Dog Gabby is a female chocolate Lab
toString() can be used with every object and allows you to get its class. To use the
Object.prototype.toString() with every object, you need to call Function.prototype.call() or
Function.prototype.apply() on it, passing the object you want to inspect as the first parameter
called thisArg.
var toString = Object.prototype.toString;
toString.call(new Date); // [object Date]
toString.call(new String); // [object String]
toString.call(Math); // [object Math]
Object represented as a string.
Returns the primitive value of the specified object.
JavaScript calls the valueOf method to convert an object to a primitive value. You rarely need to
invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an
object where a primitive value is expected.
By default, the valueOf method is inherited by every object descended from Object. Every built-
in core object overrides this method to return an appropriate value. If an object has no primitive
value, valueOf returns the object itself, which is displayed as:
[object Object]
You can use valueOf within your own code to convert a built-in object into a primitive value.
When you create a custom object, you can override Object.valueOf to call a custom method instead
of the default Object method.
You can create a function to be called in place of the default valueOf method. Your function must
take no arguments.
Suppose you have an object type myNumberType and you want to create a valueOf method for it.
The following code assigns a user-defined function to the object's valueOf method:
myNumberType.prototype.valueOf = new Function(functionText)
With the preceding code in place, any time an object of type myNumberType is used in a context
where it is to be represented as a primitive value, JavaScript automatically calls the function
defined in the preceding code.
An object's valueOf method is usually invoked by JavaScript, but you can invoke it yourself as
follows:
myNumber.valueOf()
Note: Objects in string contexts convert via the toString method, which is different from
String objects converting to string primitives using valueOf. All objects have a string
conversion, if only "[object type]". But many objects do not convert to number, boolean, or
function.
Returns value of the object or the object itself.
Creates a new object with the specified prototype object and properties.
Below is an example of how to use Object.create to achieve
classical inheritance, this is for single inheritance, which is all
that Javascript supports.
//Shape - superclass
function Shape() {
this.x = 0;
this.y = 0;
}
Shape.prototype.move = function(x, y) {
this.x += x;
this.y += y;
console.info("Shape moved.");
};
// Rectangle - subclass
function Rectangle() {
Shape.call(this); //call super constructor.
}
Rectangle.prototype = Object.create(Shape.prototype);
var rect = new Rectangle();
rect instanceof Rectangle //true.
rect instanceof Shape //true.
rect.move(); //Outputs, "Shape moved."
If you wish to inherit from multiple objects, then mixins are a possibility.
function MyClass() {
SuperClass.call(this);
OtherSuperClass.call(this);
}
MyClass.prototype = Object.create(SuperClass.prototype); //inherit
mixin(MyClass.prototype, OtherSuperClass.prototype); //mixin
MyClass.prototype.myMethod = function() {
// do a thing
};
The mixin function would copy the functions from the superclass prototype to the subclass prototype, the mixin function needs to be supplied by the user.
propertiesObject argument with Object.createvar o;
// create an object with null as prototype
o = Object.create(null);
o = {};
// is equivalent to:
o = Object.create(Object.prototype);
// Example where we create an object with a couple of sample properties.
// (Note that the second parameter maps keys to *property descriptors*.)
o = Object.create(Object.prototype, {
// foo is a regular "value property"
foo: { writable:true, configurable:true, value: "hello" },
// bar is a getter-and-setter (accessor) property
bar: {
configurable: false,
get: function() { return 10 },
set: function(value) { console.log("Setting `o.bar` to", value) }
}})
function Constructor(){}
o = new Constructor();
// is equivalent to:
o = Object.create(Constructor.prototype);
// Of course, if there is actual initialization code in the Constructor function, the Object.create cannot reflect it
// create a new object whose prototype is a new, empty object
// and a adding single property 'p', with value 42
o = Object.create({}, { p: { value: 42 } })
// by default properties ARE NOT writable, enumerable or configurable:
o.p = 24
o.p
//42
o.q = 12
for (var prop in o) {
console.log(prop)
}
//"q"
delete o.p
//false
//to specify an ES3 property
o2 = Object.create({}, { p: { value: 42, writable: true, enumerable: true, configurable: true } });
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be the prototype of the newly-created object.
Throws a TypeError exception if the proto parameter isn't null or
an object.
If specified and not undefined, an object whose enumerable own properties (that is, those properties defined upon itself and not enumerable properties along its prototype chain) specify property descriptors to be added to the newly-created object, with the corresponding property names.
the newly created object.
Defines new or modifies existing properties directly on an object, returning the object.
In essence, it defines all properties corresponding to the enumerable own properties of props on the object.
Object.defineProperties(obj, {
"property1": {
value: true,
writable: true
},
"property2": {
value: "Hello",
writable: false
}
// etc. etc.
});
NOTE: This method is part of the ECMAScript 5 standard.
Defines a new property directly on an object, or modifies an existing property on an object, and returns the object.
This method allows precise addition to or modification of a property on an object. Normal property addition through assignment creates properties which show up during property enumeration (for...in loop or keys method), whose values may be changed, and which may be deleted. This method allows these extra details to be changed from their defaults.
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.
Both data and accessor descriptor is an object with the following optional keys:
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.
A data descriptor is an object with the following optional keys:
value The value associated with the property. Can be any valid JavaScript value (number, object, function, etc) Defaults to undefined.
writable True if and only if the value associated with the property may be changed with an assignment operator. Defaults to false.
An accessor descriptor is an object with the following optional keys:
get A function which serves as a getter for the property, or undefined if there is no getter. The function return will be used as the value of property. Defaults to undefined.
set A function which serves as a setter for the property, or undefined if there is no setter. The function will receive as only argument the new value being assigned to the property. Defaults to undefined.
Bear in mind that these options are not necessarily own properties so, if inherited, will be considered too. In order to ensure these defaults are preserved you might freeze the Object.prototype upfront, specify all options explicitly, or point to null as proto property.
NOTE: This method is part of the ECMAScript 5 standard.
Freezes an object: that is, prevents new properties from being added to it; prevents existing properties from being removed; and prevents existing properties, or their enumerability, configurability, or writability, from being changed. In essence the object is made effectively immutable. The method returns the object being frozen.
Nothing can be added to or removed from the properties set of a frozen object. Any attempt to do so will fail, either silently or by throwing a TypeError exception (most commonly, but not exclusively, when in strict mode).
Values cannot be changed for data properties. Accessor properties (getters and setters) work the same (and still give the illusion that you are changing the value). Note that values that are objects can still be modified, unless they are also frozen.
NOTE: This method is part of the ECMAScript 5 standard.
The object to freeze.
Returns a property descriptor for an own property (that is, one directly present on an object, not present by dint of being along an object's prototype chain) of a given object.
This method permits examination of the precise description of a property. A property in JavaScript consists of a string-valued name and a property descriptor. Further information about property descriptor types and their attributes can be found in defineProperty.
NOTE: This method is part of the ECMAScript 5 standard.
The object in which to look for the property.
The name of the property whose description is to be retrieved.
A property descriptor is a record with some of the following attributes:
value The value associated with the property (data descriptors only).
writable True if and only if the value associated with the property may be changed (data descriptors only).
get A function which serves as a getter for the property, or undefined if there is no getter (accessor descriptors only).
set A function which serves as a setter for the property, or undefined if there is no setter (accessor descriptors only).
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.
enumerable true if and only if this property shows up during enumeration of the properties on the corresponding object.
Value of the property descriptor.
Returns an array of all properties (enumerable or not) found directly upon a given object.
Rreturns an array whose elements are strings corresponding to the enumerable and non-enumerable properties found directly upon obj. The ordering of the enumerable properties in the array is consistent with the ordering exposed by a for...in loop (or by keys) over the properties of the object. The ordering of the non-enumerable properties in the array, and among the enumerable properties, is not defined.
var arr = ["a", "b", "c"];
print(Object.getOwnPropertyNames(arr).sort()); // prints "0,1,2,length"
// Array-like object
var obj = { 0: "a", 1: "b", 2: "c"};
print(Object.getOwnPropertyNames(obj).sort()); // prints "0,1,2"
// Printing property names and values using Array.forEach
Object.getOwnPropertyNames(obj).forEach(function(val, idx, array) {
print(val + " -> " + obj[val]);
});
// prints
// 0 -> a
// 1 -> b
// 2 -> c
// non-enumerable property
var my_obj = Object.create({}, { getFoo: { value: function() { return this.foo; }, enumerable: false } });
my_obj.foo = 1;
print(Object.getOwnPropertyNames(my_obj).sort()); // prints "foo, getFoo"
If you want only the enumerable properties, see keys or use a for...in loop (although note that this will return enumerable properties not found directly upon that object but also along the prototype chain for the object unless the latter is filtered with hasOwnProperty).
Items on the prototype chain are not listed:
function ParentClass () {
}
ParentClass.prototype.inheritedMethod = function () {
};
function ChildClass () {
this.prop = 5;
this.method = function () {};
}
ChildClass.prototype = new ParentClass;
ChildClass.prototype.prototypeMethod = function () {
};
alert(
Object.getOwnPropertyNames(
new ChildClass() // ["prop", "method"]
)
)
NOTE: This method is part of the ECMAScript 5 standard.
The object whose enumerable and non-enumerable own properties are to be returned.
Array of property names.
Determines if an object is extensible (whether it can have new properties added to it).
Objects are extensible by default: they can have new properties added to them, and can be modified. An object can be marked as non-extensible using preventExtensions, seal, or freeze.
// New objects are extensible.
var empty = {};
assert(Object.isExtensible(empty) === true);
// ...but that can be changed.
Object.preventExtensions(empty);
assert(Object.isExtensible(empty) === false);
// Sealed objects are by definition non-extensible.
var sealed = Object.seal({});
assert(Object.isExtensible(sealed) === false);
// Frozen objects are also by definition non-extensible.
var frozen = Object.freeze({});
assert(Object.isExtensible(frozen) === false);
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be checked.
True when object is extensible.
Determines if an object is frozen.
An object is frozen if and only if it is not extensible, all its properties are non-configurable, and all its data properties (that is, properties which are not accessor properties with getter or setter components) are non-writable.
// A new object is extensible, so it is not frozen.
assert(Object.isFrozen({}) === false);
// An empty object which is not extensible is vacuously frozen.
var vacuouslyFrozen = Object.preventExtensions({});
assert(Object.isFrozen(vacuouslyFrozen) === true);
// A new object with one property is also extensible, ergo not frozen.
var oneProp = { p: 42 };
assert(Object.isFrozen(oneProp) === false);
// Preventing extensions to the object still doesn't make it frozen,
// because the property is still configurable (and writable).
Object.preventExtensions(oneProp);
assert(Object.isFrozen(oneProp) === false);
// ...but then deleting that property makes the object vacuously frozen.
delete oneProp.p;
assert(Object.isFrozen(oneProp) === true);
// A non-extensible object with a non-writable but still configurable property is not frozen.
var nonWritable = { e: "plep" };
Object.preventExtensions(nonWritable);
Object.defineProperty(nonWritable, "e", { writable: false }); // make non-writable
assert(Object.isFrozen(nonWritable) === false);
// Changing that property to non-configurable then makes the object frozen.
Object.defineProperty(nonWritable, "e", { configurable: false }); // make non-configurable
assert(Object.isFrozen(nonWritable) === true);
// A non-extensible object with a non-configurable but still writable property also isn't frozen.
var nonConfigurable = { release: "the kraken!" };
Object.preventExtensions(nonConfigurable);
Object.defineProperty(nonConfigurable, "release", { configurable: false });
assert(Object.isFrozen(nonConfigurable) === false);
// Changing that property to non-writable then makes the object frozen.
Object.defineProperty(nonConfigurable, "release", { writable: false });
assert(Object.isFrozen(nonConfigurable) === true);
// A non-extensible object with a configurable accessor property isn't frozen.
var accessor = { get food() { return "yum"; } };
Object.preventExtensions(accessor);
assert(Object.isFrozen(accessor) === false);
// ...but make that property non-configurable and it becomes frozen.
Object.defineProperty(accessor, "food", { configurable: false });
assert(Object.isFrozen(accessor) === true);
// But the easiest way for an object to be frozen is if Object.freeze has been called on it.
var frozen = { 1: 81 };
assert(Object.isFrozen(frozen) === false);
Object.freeze(frozen);
assert(Object.isFrozen(frozen) === true);
// By definition, a frozen object is non-extensible.
assert(Object.isExtensible(frozen) === false);
// Also by definition, a frozen object is sealed.
assert(Object.isSealed(frozen) === true);
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be checked.
True if the object is frozen, otherwise false.
Determines if an object is sealed.
An object is sealed if it is non-extensible and if all its properties are non-configurable and therefore not removable (but not necessarily non-writable).
// Objects aren't sealed by default.
var empty = {};
assert(Object.isSealed(empty) === false);
// If you make an empty object non-extensible, it is vacuously sealed.
Object.preventExtensions(empty);
assert(Object.isSealed(empty) === true);
// The same is not true of a non-empty object, unless its properties are all non-configurable.
var hasProp = { fee: "fie foe fum" };
Object.preventExtensions(hasProp);
assert(Object.isSealed(hasProp) === false);
// But make them all non-configurable and the object becomes sealed.
Object.defineProperty(hasProp, "fee", { configurable: false });
assert(Object.isSealed(hasProp) === true);
// The easiest way to seal an object, of course, is Object.seal.
var sealed = {};
Object.seal(sealed);
assert(Object.isSealed(sealed) === true);
// A sealed object is, by definition, non-extensible.
assert(Object.isExtensible(sealed) === false);
// A sealed object might be frozen, but it doesn't have to be.
assert(Object.isFrozen(sealed) === true); // all properties also non-writable
var s2 = Object.seal({ p: 3 });
assert(Object.isFrozen(s2) === false); // "p" is still writable
var s3 = Object.seal({ get p() { return 0; } });
assert(Object.isFrozen(s3) === true); // only configurability matters for accessor properties
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be checked.
True if the object is sealed, otherwise false.
Returns an array of a given object's own enumerable properties, in the same order as that provided by a for-in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).
Returns an array whose elements are strings corresponding to the enumerable properties found directly upon object. The ordering of the properties is the same as that given by looping over the properties of the object manually.
var arr = ["a", "b", "c"];
alert(Object.keys(arr)); // will alert "0,1,2"
// array like object
var obj = { 0 : "a", 1 : "b", 2 : "c"};
alert(Object.keys(obj)); // will alert "0,1,2"
// getFoo is property which isn't enumerable
var my_obj = Object.create({}, { getFoo : { value : function () { return this.foo } } });
my_obj.foo = 1;
alert(Object.keys(my_obj)); // will alert only foo
If you want all properties, even the not enumerable, see getOwnPropertyNames.
NOTE: This method is part of the ECMAScript 5 standard.
The object whose enumerable own properties are to be returned.
Array of property names.
Prevents new properties from ever being added to an object (i.e. prevents future extensions to the object).
An object is extensible if new properties can be added to it.
preventExtensions marks an object as no longer extensible, so that
it will never have properties beyond the ones it had at the time it
was marked as non-extensible. Note that the properties of a
non-extensible object, in general, may still be deleted. Attempting
to add new properties to a non-extensible object will fail, either
silently or by throwing a TypeError (most commonly, but not
exclusively, when in strict mode).
It only prevents addition of own properties. Properties can still be added to the object prototype.
If there is a way to turn an extensible object to a non-extensible one, there is no way to do the opposite in ECMAScript 5
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be made non-extensible.
Seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable.
By default, objects are extensible (new properties can be added to them). Sealing an object prevents new properties from being added and marks all existing properties as non-configurable. This has the effect of making the set of properties on the object fixed and immutable. Making all properties non-configurable also prevents them from being converted from data properties to accessor properties and vice versa, but it does not prevent the values of data properties from being changed. Attempting to delete or add properties to a sealed object, or to convert a data property to accessor or vice versa, will fail, either silently or by throwing a TypeError (most commonly, although not exclusively, when in strict mode code).
The prototype chain remains untouched.
NOTE: This method is part of the ECMAScript 5 standard.
The object which should be sealed.