There's Something Absurd About The Joker...

There are four key philosophies depicted, even if briefly or between the lines, in the 2018 movie "Joker:" nihilism, anarchy, absurdism, and existentialism. These are pretty hard to guess based on the names alone, so here's a quick rundown of each of them.

^ most people on the topic of philosophy ^

Nihilism - 
    Put simply, the idea of nothing matters and there's no point in trying to find a way to make it matter. Nihilism, as a very negative outlook on the meaning of life, can either be viewed as very pessimistic or very blunt and simple. Philosopher Friedrich Nietzche has often been associated with nihilism and the quote "God is dead[...] and we killed him." This idea of the death of God can fall under the umbrella of "values are baseless and meaning is pointless, nothing can be communicated or known," as nihilism is defined by the Intenet Encyclopedia of Philosophy's article on the topic.
    We see the ideas of nihilism play out in 'Joker,' though not necessarily in the forefront. The whole movie shows us Arther's descent into lawless disorder as he tries to search for meaning in his existence. As he continuously fails to find one and his mother is hospitalised after a stroke, he gives up and stops looking. We can see this change when he loses what's left of his humanity and shoots his mother.

Absurdism - 
    Nihilism with a sense of humour. Absurdism is the idea that there is no inherent meaning of the universe and that we, in our search for meaning, should accept that there is no humanly possible way to find it. The meaning and values of existence are logically possible but humanly impossible. However, while there is no inherent meaning, there is the possibility that one's search for meaning may in fact create its own meaning but death renders the pursuit redundant.

Existentialism - 
    In which free will allows us to govern our own meanings in life, and lead our own search in our existence. Existentialism believes that we are responsible for meaning, not a group of or any singular deities, not a government, not teachers, and not any other authority. According to ethics.org.au, "existentialism was popularised by Jean-Paul Sartre in the mid-20th century following the horrific events of World War II." Furthermore, the term 'existence precedes essence,' meaning we exist before we have meaning, was coined.

Anarchy -
   Anarchy is "[Screw] the Government" in one philosophy. It, in its basics, is scepticism of authority and power and a disbelief in the justifications that come with them. The theories support human equality and the ideal world of a community built on consensus without coercion. There are many varieties of anarchism, from political in which activists are protesting against a formed government, to theoretical in which all the beliefs are there but are understood to be an ideal.
    Anarchy plays a massive role in the latter acts of 'Joker,' as Thomas Wayne's own words are weaponised against him in the Gotham City clown riots. The uprising against Wayne only comes to light after he begins his mayoral campaign and, in retaliation to 3 of his employees being killed, calls those who can not hold their own jealous and "clowns." It's not only a rebelling force against the capitalistic platform that Wayne stands on, but the governing role he believes he deserves because of it. As the city falls into chaos, we see the much more violent side of anarchy play out through the lower class citizens in clown masks.