I am learning classes in JS, and I have tried to see how static get works. From reading about it, I thought this would work:

class Builder {
  constructor() {
    this.number = 1;
  }

  static get increaseNumber() {
    return 1 + this.number;
  }
}

const builderInstance = new Builder();

But, I get undefined when I try to run this:

console.log(builderInstance.increaseNumber);

If I remove the static keyword then it works, why do I get undefined if I use static?

I am learning classes in JS, and I have tried to see how static get works. From reading about it, I thought this would work:

class Builder {
  constructor() {
    this.number = 1;
  }

  static get increaseNumber() {
    return 1 + this.number;
  }
}

const builderInstance = new Builder();

But, I get undefined when I try to run this:

console.log(builderInstance.increaseNumber);

If I remove the static keyword then it works, why do I get undefined if I use static?

• javascript
• class
• static

Share Follow asked Nov 8, 2018 at 15:21 LeffLeff 1,4403131 gold badges111111 silver badges226226 bronze badges

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2 Answers 2

Sorted by: Reset to default 3

You get undefined because increaseNumber is static and you try to call it from an instance of Builder.

static is a keyword stating that the method will be callable by the class.

You cannot call a static method directly from the object.

here is a tutorial explaining it

class Builder {
  constructor() {
    this.number = 1;
  }

  static get increaseNumber() {
    return 1 + this.number;
  }
}

const builderInstance = new Builder();

console.log(builderInstance.increaseNumber);

---

Here is the appropriate implementation to call from an object :

An object represent an instance of a class

class Builder {
  constructor() {
    this.number = 1;
  }

  get increaseNumber() {
    return 1 + this.number;
  }
}

const builderInstance = new Builder();

console.log(builderInstance.increaseNumber);

---

As you asked me from the ment, the following snippet returns NaN, why ?

class Builder {
  constructor() {
    this.number = 1;
  }

  static get increaseNumber() {
    return 1 + this.number;
  }
}

console.log(Builder.increaseNumber);

Because you try to access to a property called number ; which is undefined and 1 + undefined equals NaN.

When calling the static method, you are not executing the constructor and so the number is not initialized.

What you can do :

class Builder {
  static get increaseNumber() {
    // We use || here to say "if it's not initialized, initialize it to 0"
    this.number = (this.number || 0) + 1;
    
    return this.number;
  }
}

console.log(Builder.increaseNumber);
console.log(Builder.increaseNumber);
console.log(Builder.increaseNumber);
console.log(Builder.increaseNumber);

Like other languages with static class members, the static keyword will create a method associated with the class, and not with an instance of the class. In other words,

you can only reach a static method using the name of the class