This agreement broke down in the last decade of the century, and there are now many scholars who believe that John did know the synoptics, especially Mark, while the hypothesis of a "signs" source has been increasingly undermined.
But theories of either complete independence of or complete dependence on the synoptics are largely rejected in current scholarship: on the one hand, elements such as distinctive Johannine language, the lengthy discourses, and the prologue on the Logos, are clearly unique to John; on the other, John clearly shares a multitude of episodes with the other three.
But the author was also familiar with non-Jewish sources: the Logos of the prologue (the Word that is with God from the beginning of creation) derives from both the Jewish concept of Lady Wisdom and from the Greek philosophers, while John 6 alludes not only to the exodus but also to Greco-Roman mystery cults, while John 4 alludes to Samaritan messianic beliefs.
Chapters 19 and 21 of John hint that "the Disciple whom Jesus loved", or "the Beloved Disciple", was an eyewitness to Jesus' ministry, but the majority of scholars are cautious of accepting this at face value.
the teachings of Jesus found in the synoptic gospels are very different from those recorded in John, and since the 19th century some scholars have argued that these discourses in Johannine style are less likely to be historical, and more likely to have been written for theological purposes.