Javascript use object as key
The The object of which the
enumerable's own properties are to be returned. An array of strings that represent all the enumerable properties of the given object. If you want
all properties—including non-enumerables—see Non-object coercion In ES5, if the argument to this method is not an object (a primitive), then it will cause a From ES2015 onwards, a non-object argument will be coerced to an object.
Specifications
Browser compatibilityBCD tables only load in the browser See alsoJavaScript is designed on a simple object-based paradigm. An object is a collection of properties, and a property is an association between a name (or key) and a value. A
property's value can be a function, in which case the property is known as a method. In addition to objects that are predefined in the browser, you can define your own objects. This chapter describes how to use objects, properties, functions, and methods, and how to create your own objects. Objects in JavaScript, just as in many other programming languages, can be compared to objects in real life. The concept of objects in JavaScript can be understood with real life, tangible objects. In JavaScript, an object is a standalone entity, with properties and type. Compare it with a cup, for
example. A cup is an object, with properties. A cup has a color, a design, weight, a material it is made of, etc. The same way, JavaScript objects can have properties, which define their characteristics. A JavaScript object has properties associated with it. A property of an object can be explained as a variable that is attached to the object. Object properties are basically the same as ordinary JavaScript variables, except for the attachment to objects. The properties of an object define the characteristics of the object. You access the properties of an object with a simple dot-notation: Like all JavaScript variables, both the object name (which could be a
normal variable) and property name are case sensitive. You can define a property by assigning it a value. For example, let's create an object named The above example could also be written using an object initializer, which is a comma-delimited
list of zero or more pairs of property names and associated values of an object, enclosed in curly braces (
Unassigned properties of an object are
Properties of JavaScript objects
can also be accessed or set using a bracket notation (for more details see property accessors). Objects are sometimes called associative arrays, since each property is associated with a string value that can be used to access it. So, for example, you could access the properties of the
An object property name can be any valid JavaScript string, or anything that can be converted to a string, including an empty string. However, any property name that is not a valid JavaScript identifier cannot use dot notation. For example, a property name that has a space or a hyphen, that starts with a number, or that is held inside a variable can only be accessed using the square bracket notation. This notation is also very useful when property names are to be dynamically determined, i.e. not determinable until runtime. Examples are as follows:
JavaScript object property names (keys) can only be strings or Symbols — all keys in the square bracket notation are converted to strings unless they are Symbols. For example, in the above code, when the key You can also access properties with a string value stored in a variable. The variable must be passed in bracket notation. In the example above, the variable
This allows accessing any property as determined at runtime:
You can use the bracket notation with
The term "own property" refers to the properties of the object, but excluding those of the prototype chain. So, the
function call myCar.make = Ford myCar.model = Mustang myCar.year = 1969 Enumerate the properties of an objectThere are three native ways to list/traverse object properties:
There is no native way to list "hidden" properties (properties in the prototype chain which are not accessible through the object, because another property has the same name earlier in the prototype chain). However, this can be achieved with the following function:
Creating new objectsJavaScript has a number of predefined objects. In
addition, you can create your own objects. You can create an object using an object initializer. Alternatively, you can first create a constructor function and then instantiate an object invoking that function in conjunction with the Using object initializersUsing object initializers is sometimes referred to as creating objects with literal notation. "Object initializer" is consistent with the terminology used by C++. The syntax for an object using an object initializer is:
where Object initializers are expressions, and each object initializer results in a new object being created whenever the statement in which it appears is executed. Identical object initializers create distinct objects that will not compare to each other as equal. Objects are created as if a call to The following statement creates an object and assigns it to the variable
The following example creates
You can also use object initializers to create arrays. See array literals. Using a constructor functionAlternatively, you can create an object with these two steps:
To define an object type, create a function for the object type that specifies its name, properties, and methods. For example, suppose you want to create an object type for cars. You want this type of object to be called
Notice the use of Now you can create an object called
This statement creates You can create any number of
An object can have a property that is itself another object. For example, suppose you define an object called
and then instantiate two new
Then, you can rewrite the definition of
To instantiate the new objects, you then use the following:
Notice that instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects Note that you can always add a property to a previously defined object. For example, the statement adds a property Using the Object.create methodObjects can also be created using the
InheritanceAll objects in JavaScript inherit from at least one other object. The object being inherited from is known as the prototype, and the inherited properties can be found in the Indexing object propertiesYou can refer to a property of an object either by its property name or by its ordinal index. If you initially define a property by its name, you must always refer to it by its name, and if you initially define a property by an index, you must always refer to it by its index. This restriction applies when you create an object and its properties with a constructor function (as we did previously with the The exception to this rule is array-like objects reflected from HTML, such as the Defining properties for an object typeYou
can add a property to a previously defined object type by using the
See the Inheritance_and_the_prototype_chain for more information. Defining methodsA method is a function associated with an object, or, put differently, a method is a property of an object that is a function. Methods are defined the way normal functions are defined, except that they have to be assigned as the property of an object. See also method definitions for more details. An example is:
where You can then call the method in the context of the object as follows:
You can define methods for an object type by including a method definition in the object constructor function. You could define a function that would format and display the properties of the previously-defined
where You can make this function a method of
to the object definition. So, the full definition of
Then you can call the
Using this for object referencesJavaScript has a special keyword,
The
Defining getters and settersA getter is a method that gets the value of a specific property. A setter is a method that sets the value of a specific property. You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties. Getters and setters can be either
When defining getters and setters using object initializers all you need to do is to prefix a getter method with
The
Please note that function
names of getters and setters defined in an object literal using "[gs]et property()" are not the names of the getters themselves, even though the Getters and setters can also be added to an object at any time after creation using the
Which of the two forms to choose depends on your programming style and task at hand. If you already go for the object initializer when defining a prototype you will probably most of the time choose the first form. This form is more compact and natural. However, if you need to add getters and setters later — because you did not write the prototype or particular object — then the second form is the only possible form. The second form probably best represents the dynamic nature of JavaScript — but it can make the code hard to read and understand. Deleting propertiesYou can remove a non-inherited property by using the
Comparing objectsIn JavaScript, objects are a reference type. Two distinct objects are never equal, even if they have the same properties. Only comparing the same object reference with itself yields true.
For more information about comparison operators, see equality operators. See also
Can an object be a key in a JavaScript map?Introduction to JavaScript Map object
An object always has a default key like the prototype. A key of an object must be a string or a symbol, you cannot use an object as a key. An object does not have a property that represents the size of the map.
How do you get a key from an object JavaScript?Use object. keys(objectName) method to get access to all the keys of object. Now, we can use indexing like Object. keys(objectName)[0] to get the key of first element of object.
How do you set a key for an object?Object keys can be dynamically assigned in ES6 by placing an expression in square brackets. Syntax: var key="your_choice"; var object = {}; object[key] = "your_choice"; console. log(object);
Can I use a variable as a key JavaScript?Use Variable as Key for Objects in JavaScript
We will set a variable varr and enter the key key in this instance. Later we will define an object obj and set the unique key in obj[varr] . var varr = "key"; var obj = {}; obj[varr] = 42; console. log(obj.
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