JavaScript Object Constructors
Object Constructor Functions
Sometimes we need to create many objects of the same type.
To create an object type we use an object constructor function.
It is considered good practice to name constructor functions with an upper-case first letter.
Object Type Person
function Person(first, last, age, eye) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eye;
}
Try
it yourself »Note:
In the constructor function, this has no value.
The value of this will become the new object when
a new object is created.
See Also:
Now we can use new Person()
to create many new Person objects:
Example
const myFather = new Person("John", "Doe", 50, "blue");
const myMother = new Person("Sally", "Rally", 48, "green");
const mySister = new Person("Anna", "Rally", 18, "green");
const mySelf = new Person("Johnny", "Rally", 22, "green");
Try
it yourself »
Property Default Values
A value given to a property will be a default value for all objects created by the constructor:
Example
function Person(first, last, age, eyecolor) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eyecolor;
this.nationality = "English";
}
Try it Yourself »
Adding a Property to an Object
Adding a property to a created object is easy:
Note:
The new property will be added to myFather. Not to any other Person Objects.
Adding a Property to a Constructor
You can NOT add a new property to an object constructor:
To add a new property, you must add it to the constructor function prototype:
Constructor Function Methods
A constructor function can also have methods:
Example
function Person(first, last, age, eyecolor) {
this.firstName = first;
this.lastName = last;
this.age = age;
this.eyeColor = eyecolor;
this.fullName = function() {
return this.firstName + " " + this.lastName;
};
}
Try it Yourself »
Adding a Method to an Object
Adding a method to a created object is easy:
Note:
The new method will be added to myMother. Not to any other Person Objects.
Adding a Method to a Constructor
You cannot add a new method to an object constructor function.
This code will produce a TypeError:
Example
Person.changeName = function (name) {
this.lastName = name;
}
myMother.changeName("Doe");
TypeError: myMother.changeName is not a function
Adding a new method must be done to the constructor function prototype:
Example
Person.prototype.changeName = function (name) {
this.lastName = name;
}
myMother.changeName("Doe");
Try it Yourself »
Note:
The changeName() function assigns the value of name
to the person's lastName property, substituting this with myMother.
Built-in JavaScript Constructors
JavaScript has built-in constructors for all native objects:
new Object() // A new Object object
new Array() // A new Array object
new Map() // A new Map object
new Set() // A new Set object
new Date() // A new Date object
new RegExp() // A new RegExp object
new Function() // A new Function object
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Note:
The Math() object is not in the list. Math is a global object. The new keyword cannot be used on
Math.
Did You Know?
Use object literals {} instead of new Object().
Use array literals [] instead of new Array().
Use pattern literals /()/ instead of new RegExp().
Use function expressions () {} instead of new Function().
Example
""; // primitive string
0; // primitive number
false; // primitive boolean
{}; // object object
[]; // array object
/()/ // regexp object
function(){}; // function
Try it Yourself »
Complete Object Reference
For a complete reference, go to our:
Complete JavaScript Object Reference.
The reference contains descriptions and examples of all Object Properties and Methods.
