create object notes

Author: gambitier

session 1/chapters/1. objects.md

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+# Objects in JavaScript
+
+An object in JavaScript is a collection of key-value pairs. Each key-value pair is called as a property. A property can be a function, an array, an object itself or any primitive data type i.e. integer, string, etc. Functions in object are called as methods.
+
+Here firstName, lastName, and fullName are properties of the same object i.e. human. firstName is the key and Virat is the value of the property.
+
+```JavaScript
+var human = {
+	firstName: "Virat",
+	lastName: "Kohli",
+	age: 30,
+	fullName: function(){
+		return this.firstName + " " + this.lastName
+	},
+    otherInformation: {
+        adress:{
+            city: 'Mumbai',
+            street: 'street 1'
+        },
+        phone: '+91-312452345'
+    }
+}
+
+console.log(human.fullName()); //Virat Kohli
+```
+
+**Properties of the object can be accessed using**
+
+1. Dot notation.
+
+   e.g. human.fullName(), human.firstName, human.otherInformation.adress
+
+2. Square bracket
+
+   e.g.
+
+   ```
+   human["firstName"]; //Output: Virat
+   human["fullName"](); //Output: Virat Kohli
+   ```
+
+   ```
+   var firstNameProperty = "firstName";
+   console.log(human[firstNameProperty]) // Output: Virat
+   ```
+
+   **Note**: Above method of using variable to access property names cannot be used to access properties of the object using dot notation.
+
+   ```
+   Console.log(human.firstNameProperty) //Output: undefined
+   ```
+
+New properties can be added using the dot notation as shown below:
+
+```JavaScript
+human.age = 27
+human.getAge = function(){
+	return this.age;
+}
+```
+
+An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string.
+
+**However**, any property name that is not a valid Javascript identifier (for example, a property name that has a space or a hyphen, or that starts with a number) can only be accessed and added to the object property using the square bracket notation
+
+```JavaScript
+human["date of birth"] = "Nov 28";
+human[12] = 12;
+human.12 = 12; //gives error
+
+console.log(human.12); //Gives error
+console.log(human[12]); //Output: 12
+```
+
+![jsPropsName](../images/objectProps.jpg)
+
+## Delete a property from an object
+
+To delete a property from an object we can use the delete operator. You cannot delete properties that were inherited, nor can you delete properties with their attributes set to configurable.
+
+‘delete’ operator returns true if the delete was successful. It also return true if the property to delete was non-existent or the property could not be deleted.
+
+```
+delete human.firstName; // return true
+```
+
+Let’s see what happens if we try to call fullName method which uses both the firstName and lastName property of human object.
+
+`console.log(human.fullName());// undefined Kohli`
+
+Output is undefined because we were trying to access firstName property of human object which does not exists.