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C # tutorial
Properties are the most important part of any JavaScript object
Properties are the values associated with a JavaScript object.
A JavaScript object is a collection of unordered properties.
Properties can usually be changed, added, and deleted, but some are read only.
The syntax for accessing the property of an object is:
or
or
The expression must evaluate to a property name.
person.firstname + " is " + person.age + " years old.";
person["firstname"] + " is " + person["age"] + " years old.";
The JavaScript for...in
statement loops through the properties of an object.
Syntax
The block of code inside of the for...in
loop will be executed once for each property.
Looping through the properties of an object:
const person = {
fname:" John",
lname:" Doe",
age: 25
};
for (let x in person) {
txt += person[x];
}
You can add new properties to an existing object by simply giving it a value.
Assume that the person object already exists - you can then give it new properties:
person.nationality = "English";
The delete
keyword deletes a property from an object:
const person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};
delete person.age;
or delete person["age"];
const person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};
delete person["age"];
The delete
keyword deletes both the value of the property and the property itself.
After deletion, the property cannot be used before it is added back again.
The
delete
operator is designed to be used on object properties. It has no effect on
variables or functions.
The delete
operator should not be used on predefined JavaScript object
properties. It can crash your application.
Values in an object can be another object:
myObj = {
name:"John",
age:30,
cars: {
car1:"Ford",
car2:"BMW",
car3:"Fiat"
}
}
You can access nested objects using the dot notation or the bracket notation:
myObj.cars.car2;
or:
myObj.cars["car2"];
or:
myObj["cars"]["car2"];
or:
let p1 = "cars";
let p2 = "car2";
myObj[p1][p2];
Values in objects can be arrays, and values in arrays can be objects:
const myObj =
{
name: "John",
age: 30,
cars: [
{name:"Ford",
models:["Fiesta", "Focus", "Mustang"]},
{name:"BMW", models:["320", "X3", "X5"]},
{name:"Fiat", models:["500", "Panda"]}
]
}
To access arrays inside arrays, use a for-in loop for each array:
for (let i in myObj.cars) {
x += "<h1>" + myObj.cars[i].name
+ "</h1>";
for (let j in myObj.cars[i].models) {
x += myObj.cars[i].models[j];
}
}
All properties have a name. In addition they also have a value.
The value is one of the property's attributes.
Other attributes are: enumerable, configurable, and writable.
These attributes define how the property can be accessed (is it readable?, is it writable?)
In JavaScript, all attributes can be read, but only the value attribute can be changed (and only if the property is writable).
( ECMAScript 5 has methods for both getting and setting all property attributes)
JavaScript objects inherit the properties of their prototype.
The delete
keyword does not delete inherited properties, but if you delete a
prototype property, it will affect all objects
inherited from the prototype.