The descendants of Judah were Er and Onan, but Er and Onan died in the land of Canaan.

The sons of Judah: Er, Onan, Shelah, Perez, and Zerah (but Er and Onan died in the land of Canaan). The sons of Perez were Hezron and Hamul.

She became pregnant again and had another son, whom she named Onan.

Then Judah said to Onan, “Have sexual relations with ¹  your brother’s wife and fulfill the duty of a brother-in-law to her so that you may raise ²  up a descendant for your brother.” ³ 

But Onan knew that the child ¹  would not be considered his. ²  So whenever ³  he had sexual relations with ⁴  his brother’s wife, he withdrew prematurely ⁵  so as not to give his brother a descendant.

The sons of Judah: Er, Onan, and Shelah. These three were born to him by Bathshua, ¹  a Canaanite woman. Er, Judah’s firstborn, displeased the Lord, so the Lord killed him. ²