Did you know that Jesus' transfiguration appears in only three of the four Gospels?
Matthew: "1And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, 2And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light." (Matthew 17:1-2)
Mark: "And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them. 3And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them." (Mark 9: 2-3)
Luke: "28And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray. 29And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering." (Luke 9:28-29)
The Gospel of John does not feature the transfiguration scene, even though he was present in all three accounts.
Because for John, Jesus' glory was always manifest.
In the first chapter of his Gospel, John God's own Son, his glory beheld by all.
How did John know so clearly and consistently about Jesus' glory.
Because he knew how much Jesus loved him:
"Now there was leaning on Jesus' bosom one of his disciples, whom Jesus loved." (John 13:23)
"When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he saith unto his mother, Woman, behold thy son!" (John 19:26)
"Then she runneth, and cometh to Simon Peter, and to the other disciple, whom Jesus loved, and saith unto them, They have taken away the Lord out of the sepulchre, and we know not where they have laid him." (John 20:2)
"Therefore that disciple whom Jesus loved saith unto Peter, It is the Lord. Now when Simon Peter heard that it was the Lord, he girt his fisher's coat unto him, (for he was naked,) and did cast himself into the sea." (John 21:7)
"Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?" (John 21:20)
When we know how much Jesus loves us, how can we not behold his glory?