How did Yehudah (Judah) know what to command Onan to do in Genesis 38:8 if it had not yet been written down formally as a command?

Answer From The Annual Reading Schedule - Week 10 Notes For Genesis: It is verses like this that prove beyond a doubt that it does not matter what was written down yet – laws existed even before they were written down formally. It doesn't have to have yet been written down for the law to exist. It is this, along with other laws, that Avraham (Abraham) had been said to have kept in Genesis 26:5. Many of the laws later written down were first passed down orally throughout the generations.