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Basics
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First, we can create new objects in two different ways We can use the builtin Object type with the new keyword, or we can use an object literal An object literal is a string that de nes the object and is begun with a left curly brace ({) and ended with a right curly brace (}) Listing 15 demonstrates using these two methods to create two new object instances
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Listing 15 Creating New Object Instances
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var myCar = new Object(); var myCar2 = { };
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1: Programming with JavaScript
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These objects already have a function available just because they re objects The toString function is attached to the object s prototype (we explain what a prototype is in a bit), and therefore it is subsequently available on all instances that inherit from Object, which is everything Besides having the toString function, which would require an override to make it output anything interesting, these objects are a bit plain because they have no properties assigned We can assign properties to an object using either the Object Literal notation or through a dot notation Listing 16 shows both ways as we add the make and model properties to the two object instances we created in Listing 15
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Listing 16 Object Properties
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var myCar = new Object(); myCarmake = 'Ford'; myCarmodel = 'Explorer'; var myCar2 = { make: "Ford", model: "Explorer" };
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JavaScript stores an object s properties using an associative array, which is an array that is accessed by key rather than index The dot notation that we used in Listing 16 is just another way of accessing the values in the associative array We could have just as easily added the properties using array syntax Listing 17 demonstrates this concept
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Listing 17 Object Properties as Associative Arrays
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var myCar = new Object(); myCarmake = 'Ford'; myCar["model"] = 'Explorer'; alert (myCarmake === myCar["make"]); // alerts true
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NOTE
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Associative Arrays
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Associative arrays are a great way of accessing a property on an object by name Whenever you re tempted to use an eval statement to access the property on an object, you can most likely access it by array position instead
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Strings
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Because objects store their properties in an associative array, we can iterate over the properties using a for in loop Listing 18 demonstrates this concept as we iterate over the properties of our myCar object
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Listing 18 Using a for in Loop
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var myCar = new Object(); myCarmake = 'Ford'; myCar["model"] = 'Explorer'; myCaryear = 2003; myCarmileage = 60000; var propValues; for (var propName in myCar) { propValues = propValues + " " + myCar[propName]; } alert (propValues); / alerts "Ford Explorer 2003 60000 ";
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Just as we de ned properties, we can de ne functions Listing 19 adds the print function to our myCar object
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Listing 19 Object Functions
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myCarprint = function() { alert (thismake + " " + thismodel); }; myCarprint(); // alerts 'Ford Explorer'
We could have done all of this using the Object Literal notation, too Listing 110 shows how to create the same object using Object Literal notation
Listing 110 Object Literal Notation
var myOtherCar = { make: "Ford", model: "Explorer", year: 2003, mileage: 60000 print: function() { alert (thismake + " " + thismodel); } } myOtherCarprint // alerts 'Ford Explorer'
1: Programming with JavaScript
Adding properties and functions to an existing object is useful in many cases, but has serious drawbacks because those properties and functions are added only to that particular instance If we wanted to create another object that has the same properties and functions, we would have to re-add them to the new object instance Fortunately, JavaScript provides other ways of attaching properties and functions to an object so that they have to be de ned only one time We cover this mechanism in the Object-Oriented JavaScript Programming section of this chapter
NOTE
Expando Properties
Adding properties and functions to an object in the manner we just discussed is called adding an Expando property Expando properties tend to be slow performing when compared to custom objects; so although they have some useful purposes, their use should be limited to those situations where you have no choice but to use them Instead, you should use the custom object technique we cover in the ObjectOriented JavaScript Programming section of this chapter
After we add a property or a function to an object, we can set its value back to null by assigning null to it, as shown here:
myCarprint = null;
This is useful, but if we iterate over this object using the for in loop, print will still be included in the loop If you don t want the property at all anymore and want to completely remove it, you can delete it from the object using the delete command as follows:
delete myCarprint;
Deleting a property removes it from the object so that it will no longer be returned from the for in loop, and if accessed, it returns undefined
JavaScript Object Notation (JSON)
JSON is a data interchange format similar to XML in purpose, but lighter than XML when compared on the number of characters needed to de ne an object with the same content Listing 111 shows the JSON and XML needed to create an object with the same content