How `self` in Python Is Different From `this` in JavaScript

Knowing their differences can help you write more efficient and bug-free code.

Published in

Level Up Coding

4 min readMar 22, 2023

self in Python and this in JavaScript are both used to refer to the current object instance within a method or function. However, the syntax and behavior of these keywords differ between the two languages.

`self` in Python

In Python, self is a reference to the current object instance within a class method. When we define a class in Python, we can define methods that are associated with that class. Within these methods, we can use self to refer to the current instance of the class. Here’s an example:

class MyClass:
  def __init__(self, value):
    self.value = value
  
  def print_value(self):
    print(self.value)


my_instance = MyClass(12)
my_instance.print_value()

In this example, we define a class called MyClass with an attribute value and a method print_value. Within the print_value method, we use self.value to refer to the value attribute of the current instance of the class. When we create an instance of the class and call the print_value method, the output will be 12.

In Python, self is not a reserved keyword, but rather a convention that is widely used in class methods to refer to the current object instance. We could technically use any variable name instead of self, but using self is a convention that makes our code more readable and consistent.

`this` in JavaScript

In JavaScript, this is a reference to the current object within a function or method. When we define an object in JavaScript, we can define methods that are associated with that object. Within these methods, we can use this to refer to the current instance of the object. Here’s an example:

const myObject = {
  value: 12,
  printValue() {
    console.log(this.value);
  }
};

myObject.printValue();

In this example, we define an object called myObject with a property value and a method printValue. Within the printValue method, we use this.value to refer to the value property of the current instance of the object. When we call the printValue method on the myObject instance, the output will be 12.

In JavaScript, this is dynamically bound, which means that its value depends on how the function is called. The value of this is determined by the object that is used to invoke the function (exclude arrow functions). If a function is called without an object, the value of this will be the global object (window in a browser, global in Node.js). This behavior can sometimes lead to confusion and errors in JavaScript code.

So the arrow function was introduced in ES6, It doesn’t have a this, and if you access this in an arrow function, it returns the this of the nearest non-arrow function. For more information see:

7 Differences Between Arrow Functions and Traditional Functions

Can arrow functions completely replace traditional functions?

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Key differences

While self in Python and this in JavaScript are used to refer to the current object instance, there are several key differences between the two keywords:

Syntax: In Python, we use self as the first parameter of a class method. In JavaScript, we use this within the body of a function or method.
Dynamic binding: In JavaScript, this is dynamically bound, meaning that its value depends on how the function is called. In Python, self is statically bound, meaning that its value is determined by the instance of the class that the method is called on.
Reserved keyword: self in Python is not a reserved keyword, but rather a convention. this in JavaScript is a reserved keyword and cannot be used as a variable name.
Error-proneness: Due to the dynamic binding of this in JavaScript, it can sometimes lead to errors and confusion, especially when working with nested functions and callbacks. self in Python is less error-prone and provides more clarity in code.

Conclusion

In conclusion, self in Python and this in JavaScript are both used to refer to the current object instance within a method or function. While the syntax and behavior of these keywords differ between the two languages, understanding their differences can help you write more effective and error-free code.

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