When she heard the news, she was beside herself with joy.

was 是 be 动词的过去式，表示"处于某种状态"。 beside 是介词，本义是"在...旁边"，在这里与 herself 构成习语。 herself 是反身代词，指"她自己"。 beside oneself 是固定短语，字面意思是"在自己身体之外"，引申为"极度激动、失去自制"的状态，常与with joy/anger等情绪词连用。