John Gill Commentary Psalms 118:21

"I will give thanks unto thee; for thou hast answered me, And art become my salvation." — Psalms 118:21 (ASV)

I will praise you, for you have heard me
Here the psalmist reassumes his part in this song, and determines to praise the Lord for hearing him when in distress, and when he was encompassed with his enemies, and for delivering him out of their hands;

and art become my salvation ;
the author of it, and therefore deserving of praise; and who is no other than the Messiah Jesus, who is described in the next verse.