What could be causing this error I'm receiving: instance versus prototype difference?

Question

What could be causing this error I'm receiving: instance versus prototype difference?

Can someone help me understand the difference between .prototype and regular instances in JavaScript? I am confused about why this code is not working and getting a type error: "undefined is not a function". I am experimenting with the Ninja() class and trying to compare it to .prototype and the first instance. Then I want to create a new instance of Ninja() called ninja and observe the differences.

     function Ninja() {
     this.swingSword = function() {
     return true;
     };
     }

     Ninja.prototype.swingSword = function() {
     return false;
     };

    var ninja = new Ninja;
    console.log(Ninja.prototype.swingSword());
    console.log(Ninja.swingSword());
    console.log(ninja.swingSword());
    console.log(ninja.prototype.swingSword());

javascript class inheritance prototype instance

Answer 1

Answer №1

Prototype is a specific property of the Function object. Since all function objects inherit from Function, they all possess this property. In the case mentioned, Ninja is a function, while ninja is an object created by Ninja. As a result, ninja does not have a prototype property, leading to a JavaScript response of undefined. Attempting to call swingSword on undefined is not feasible, hence the error message.

You can verify this by using the following code snippet:

console.log({}.toString.call(Ninja));
# [object Function]
console.log({}.toString.call(ninja));
# [object Object]
console.log(Ninja.prototype);
# { swingSword: [Function] }
console.log(ninja.prototype);
# undefined

It is important to note that the method swingSword is only defined on Ninja.prototype and not on Ninja itself. Therefore, when invoking Ninja.swingSowrd(), JavaScript will first search for it within Ninja and then in its parent's prototype, which in this case would be Function.prototype. Since lookups on Ninja do not involve Ninja.prototype, an error will occur on that particular line as well.

Answer 2

Prototype is a specific property of the Function object. Since all function objects inherit from Function, they all possess this property. In the case mentioned, Ninja is a function, while ninja is an object created by Ninja. As a result, ninja does not have a prototype property, leading to a JavaScript response of undefined. Attempting to call swingSword on undefined is not feasible, hence the error message.

You can verify this by using the following code snippet:

console.log({}.toString.call(Ninja));
# [object Function]
console.log({}.toString.call(ninja));
# [object Object]
console.log(Ninja.prototype);
# { swingSword: [Function] }
console.log(ninja.prototype);
# undefined

It is important to note that the method swingSword is only defined on Ninja.prototype and not on Ninja itself. Therefore, when invoking Ninja.swingSowrd(), JavaScript will first search for it within Ninja and then in its parent's prototype, which in this case would be Function.prototype. Since lookups on Ninja do not involve Ninja.prototype, an error will occur on that particular line as well.

Answer 3

Answer №2

Attempting to execute Ninja.swingSword(); will result in an error because the function is not defined prior to the Ninja() constructor/function being called. However, you can achieve the desired outcome by using new Ninja.swingSword();, which will return a value of true.

It's worth noting that ninja.prototype is undefined since it's an instance of the Ninja class, and any prototype variables are directly assigned to ninja. During construction, this.swingSword is set to a new function that returns true. Consequently, calling ninja.swingSword(); will provide a result of true rather than false, as one might anticipate with the prototype function.

Answer 4

Attempting to execute Ninja.swingSword(); will result in an error because the function is not defined prior to the Ninja() constructor/function being called. However, you can achieve the desired outcome by using new Ninja.swingSword();, which will return a value of true.

It's worth noting that ninja.prototype is undefined since it's an instance of the Ninja class, and any prototype variables are directly assigned to ninja. During construction, this.swingSword is set to a new function that returns true. Consequently, calling ninja.swingSword(); will provide a result of true rather than false, as one might anticipate with the prototype function.

Answer 5

Answer №3

When it comes to the Ninja function, it does not actually have a swingSword method. Instead, the Ninja.prototype is where you will find this method. A function in JavaScript is essentially an object like many others, but one key difference is that you can create instances of it using the new keyword and it also comes with a built-in prototype member.

Let's say we have an object named john:

var john = {
  name: "John",
  location: {
    city: "City A",
    state: "State B"
  }
}

From this example, it becomes clear that john does not have a direct property called john.city, but it does have john.location.city as a nested property.

If you want to learn more about constructor functions and prototype in JavaScript, check out this link.

Answer 6

When it comes to the Ninja function, it does not actually have a swingSword method. Instead, the Ninja.prototype is where you will find this method. A function in JavaScript is essentially an object like many others, but one key difference is that you can create instances of it using the new keyword and it also comes with a built-in prototype member.

Let's say we have an object named john:

var john = {
  name: "John",
  location: {
    city: "City A",
    state: "State B"
  }
}

From this example, it becomes clear that john does not have a direct property called john.city, but it does have john.location.city as a nested property.

If you want to learn more about constructor functions and prototype in JavaScript, check out this link.

What could be causing this error I'm receiving: instance versus prototype difference?

Answer №1

Answer №2

Answer №3

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