The Book of John is included in the Bible, making it Scripture along with the rest of the books. Scripture is simply God’s Word written down so everything in the Bible constitutes Scripture.
As John 1 continues further down: John 1:9–18 There was the true Light which, coming into the world, enlightens every man. He was in the world, and the world was made through Him, and the world did not know Him. He came to His own, and those who were His own did not receive Him. But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God. And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth. John *testified about Him and cried out, saying, “This was He of whom I said, ‘He who comes after me has a higher rank than I, for He existed before me.’” For of His fullness we have all received, and grace upon grace. For the Law was given through Moses; grace and truth were realized through Jesus Christ. No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him.