The call arr[2]() is syntactically the good old obj[method](), in the role of obj we have arr, and in the role of method we have 2.

So we have a call of the function arr[2] as an object method. Naturally, it receives this referencing the object arr and outputs the array:

let arr = ["a", "b"];

arr.push(function() {
  alert( this );
})

arr[2](); // a,b,function(){...}

The array has 3 values: initially it had two, plus the function.