JS Daily 2 - this in Arrow Function

What is the output of following code?

const obj = {
    a : 10,
    b : () => {
        console.log(this.a);
    }
}

obj.b();

Arrow function does not have its own this. Arrow functions establish this based on the scope the Arrow function is defined within(not invoked). Here the arrow function is defined in global context. Therefore, this points to window object in browsers.

Since we have not set a property a for window object, line 4 prints undefined in console.

Just for fun, can you find out the output of below code?

function fun() {
  const obj = {
    a: 10,
    b: () => {
      console.log(this.a);
    },
  };
  return obj;
}

const obj = fun.apply({ a: 20 });
obj.b();

Note: this is based on where the arrow function is defined, NOT invoked.