Declare in JavaScript - ECMAScript 2021

To declare a class in ECMAScript 2021, use the 'class' keyword followed by the class name. Here's an example:

class Animal {
  constructor(name) {
    this.name = name;
  }
  
  speak() {
    console.log(${this.name} makes a noise.);
  }
}
let cat = new Animal('Fluffy');

cat.speak();

In this example, the Animal class has a constructor that sets the name property, and a speak() method that logs a message to the console. To create a new instance of the class, we use the new keyword with the class name, passing in any required arguments to the constructor.

Another example:

class Person {
  constructor(name, age, gender) {
    this.name = name;
    this.age = age;
    this.gender = gender;
  }
  
  introduce() {
    console.log(Hi, my name is ${this.name}, I am ${this.age} years old and I identify as ${this.gender}.);
  }
}
let john = new Person('John', 30, 'male');

john.introduce();

In this example, the Person class has a constructor that sets the name, age and gender properties, and an introduce() method that logs a message to the console. To create a new instance of the class, we use the new keyword with the class name, passing in the required arguments to the constructor.