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What is # in JavaScript

Unraveling the Mystery of the # in JavaScript

What Does the # Signify in JavaScript?

In JavaScript, the # symbol, also known as a hash or pound sign, holds a special significance. It is used to denote private fields in JavaScript classes. Now, you might be thinking, what are private fields?

In simple terms, private fields are features that can only be accessed within the class they are defined in. They are not visible or accessible from outside the class. Think of a private field like a secret diary, which you keep hidden from others. It's not that you're hiding something, but it's simply a personal space that you don't want others to intrude on. Similarly, a private field in a JavaScript class is a space that is kept exclusive to that class.

Private Fields in JavaScript Classes

To understand this better, let's see how we can use the # in a JavaScript class with an actual code example. Let's consider a class Person. This class has a public field name and a private field age. Here's how we can declare this class:

class Person {
  name = '';
  #age = 0;

  constructor(name, age) { = name;
    this.#age = age;

  greet() {
    console.log(`Hello, my name is ${} and I am ${this.#age} years old.`);

In the code above, #age is a private field. We can create an instance of the Person class and call the greet method like so:

let john = new Person('John', 25);
john.greet();  // outputs: Hello, my name is John and I am 25 years old.

Even though the #age field is private, we can access it within the greet method because this method is part of the Person class. However, if we try to access the #age field directly from the john instance, we will get an error:

console.log(john.#age);  // outputs: SyntaxError: Private field '#age' must be declared in an enclosing class

The Need for Private Fields

You might be wondering why we need private fields in the first place. After all, isn't it easier to just make everything public?

Think of a class as a box of chocolates. Some chocolates are meant to be shared with everyone. These are like public fields. But there might be some chocolates that you want to keep for yourself. These are akin to private fields.

By making a field private, we ensure that it can only be accessed and modified within the class it belongs to. This helps us maintain the integrity of our data and protect it from unwanted manipulation from outside the class.

JavaScript # and Other Languages

The concept of private fields is not unique to JavaScript. Most object-oriented programming languages, like Java or C++, have a similar concept. However, the syntax varies from language to language. In JavaScript, we use the # symbol to denote private fields. This is different from other languages where keywords like private or protected are used.

By introducing the # symbol for private fields, JavaScript has made it easier to distinguish between private and public fields at a glance.


Cracking the code of the # in JavaScript is like unlocking a secret door. It's a doorway that leads to private fields, a feature that helps us protect our data and maintain the integrity of our classes. Like a secret diary or a special box of chocolates, private fields offer a space of exclusivity within our classes.

So the next time you see the # symbol in JavaScript, remember, it's not just a symbol. It's a guardian, a gatekeeper, and a protector of private fields. It's a testament to JavaScript's commitment to providing developers with the tools they need to write clean, secure, and efficient code. So go ahead, explore the world of private fields, and let the # be your guide!