In ECMAScript 2021 and previous versions, constructors are special functions that are used to create and initialize objects created from a class. Constructors can have parameters that can be used to set the initial properties of the instance.
Here is an example of a simple constructor function in JavaScript:
class Person {
constructor(name, age) {
this.name = name;
this.age = age;
}
}
In this example, we have defined a Person class with a constructor that takes two parameters: name and age. Inside the constructor, we use the keyword this to refer to the current instance of the Person class, and we set the name and age properties of the instance accordingly.
We can create new instances of the Person class using the new keyword, as follows:
const person1 = new Person('John', 30);
const person2 = new Person('Jane', 25);
In this example, we have created two instances of the Person class: person1 and person2. The first instance has a name of "John" and an age of 30, while the second instance has a name of "Jane" and an age of 25.
If we want to add methods to our Person class, we can simply define them inside the class, like this:
class Person {
constructor(name, age) {
this.name = name;
this.age = age;
}
sayHello() {
console.log(Hello, my name is ${this.name} and I am ${this.age} years old.);
}
}
In this example, we have added a sayHello method to our Person class. This method simply logs a message to the console that includes the name and age properties of the current instance.
We can call this method on any instance of the Person class, like this:
person1.sayHello(); // logs "Hello, my name is John and I am 30 years old."
person2.sayHello(); // logs "Hello, my name is Jane and I am 25 years old."
This is just a simple example of how to use constructors in JavaScript. Constructors can be used to create more complex classes and objects with more properties and methods.