גַּם־מִיּוֹם֙ אֲנִ֣י ה֔וּא וְאֵ֥ין מִיָּדִ֖י מַצִּ֑יל אֶפְעַ֖ל וּמִ֥י יְשִׁיבֶֽנָּה׃ (ס)
Yea, since the day was I am He, and there is none that can deliver out of My hand; I will work, and who can reverse it?
Rashi on Isaiah
Even before the day I am He Not only that day was I alone, but even before it became day I am He alone.
Ibn Ezra on Isaiah
Before the day was, etc. I was God before the existence of day. Therefore, the word גם yea is added.