Programming Fundamentals › Object-Oriented Programming

this / self

The reference to the current object inside a method.

Also known as: this, self, this keyword

Inside a method, this (JavaScript, Java, C#) or self (Python) is the object the method was called on. It’s how the method reaches that object’s own data.

class Counter:
    def __init__(self):
        self.count = 0
    def increment(self):
        self.count += 1          # "my" count

a = Counter()
a.increment()                    # self is a

class Counter {
  count = 0;
  increment() { this.count++; }
}

In Python, self is an explicit first parameter of each method. In JavaScript, this is implicit.

The JavaScript surprise

In JavaScript, this depends on how the function is called, not where it’s written:

const c = new Counter();
c.increment();                  // this is c: works
const f = c.increment;
f();                            // this is undefined or the global: breaks
setTimeout(c.increment, 100);   // same problem

Fixes:

setTimeout(() => c.increment(), 100);       // call it as a method inside an arrow function
setTimeout(c.increment.bind(c), 100);       // bind it to c

Arrow functions don’t have their own this. They use the one from where they were written, which is why they suit callbacks inside methods (this binding).

Common mistakes

• Forgetting self. in Python: count += 1 inside a method makes a local variable, not the attribute.
• Losing this when passing methods around as callbacks.
• Using this in a plain function or at the top level, where it may be undefined.

In Python, the name self is a convention, not a keyword, but always use it.