There are so manuscripts that have been published of this book. It is said to be “a midrash that covers Biblical history, from creation until the Judges period. Named after an ancient text referred to in Joshua 10:12 and II Samuel 1:17, it is also referred to as the Toledot Adam or Divrei haYamim heArukh. The introduction describes how a Roman general discovered the work during the destruction of the Second Temple, but scholarship has long maintained that the work is medieval.” https://www.sefaria.org/Sefer_HaYashar_(midrash)?tab=contents

Whatever it is, it’s at least an interesting read. I decided to grab a copy to see for myself what it is. From what I can tell, it follows Scripture very well with adding in a lot of details that Scripture doesn’t give us. Many of these details are confirmed in other Rabbinic texts. I started creating a timeline chart to see if it held up to the Scriptures and it does. I don’t know if the extra stories and events in this book are true, but I know after careful study of the timeline, they are at least possible. The only time I saw any conflict with Scripture was over the dates of Moses leaving Egypt and returning to Egypt for the Exodus. In my chart, I used the dates from Scripture for Moses.

Here is a timeline chart with the highlights I gathered from the Book of Jasher.

Below is a timeline I created years ago of the ages of Adam to Jacob as listed from Scripture. It does not contradict with the Book of Jasher.