Milan Kundera’s Novel The Joke abounds in existentialist vocabulary and themes. His main character’s narrative is loaded with such expressions as choice and existence, self-deception or freedom, which are sometimes even set in Italics. These are not just words, that Ludvik uses in his narrative, they are also made themes in themselves as well as reoccurring motifs – for example that of the ‘destruction of façades’. These themes are important, from the very beginning as we have the basic condition of someone ‘crossing the border’, which is not altogether dissimilar to Roquentin’s predicament in Sartre’s La Nausée.
All situations, that Kundera’s characters find themselves in are existential, too, in such a way, that they require choices and a wholly new perspective on the past after the destruction of old value systems.
It is therefore obvious that existentialist ideas must have influenced and inspired Kundera, and as Existentialism is in itself a philosophy that focuses greatly on perception and the possibility of self-knowledge, one could even propose to say, that its ideas are always also central to modern novel writing; especially when we look at psychological novels and first-person narratives.
However, (I would argue that) The Joke is by no means an existentialist novel, as its approach to ideas is by no means direct, but playful as well as critical. This is why it is impossible, to say, that for example, the novel reflects on the nature of self-deception, and its main character is in ‘bad faith’. Such an approach, does not do the novel justice, and at this point it may well be worth remembering, that its author, too, resists such an attempt:
“My disgust for those who reduce a work to its ideas: my revulsion at being dragged into what they call ‘discussions of ideas.’ My despair at this era befogged with ideas and indifferent to works.” (Art of the Novel, p. 131).
Now, let me first quickly define, what I understand by ‘existentialist novel’ before we can then look at a few actual passages and maybe reach the above verdict.
Existentialists, like Sartre, have always used fiction to convey their ideas. According to Edith Kern, Simone de Beauvoir even thought of the novel as the most appropriate means of presenting existentialist thought: “The novel seemed to her [Simone de Beauvoir] particularly suited to the expression of existential insights which would seem contradictory if they were to be presented categorically and systematically.” (Kern, p. 11).
 E.g. on page 46 “not a matter of choice but of essence”
 "BORDER. It takes so little, so infinitely little, for a person to cross the border beyond which everything loses meaning: love, convictions, faith, history. Human life - and herein lies its secret - takes place in the immediate proximity of that border, even in direct contact with it; it is not miles away but a fraction of an inch." (Milan Kundera in ‘Art of the Novel’, p 124.)