The phrase "in a heartbeat" means immediately, without any hesitation or delay. It is an idiomatic expression that conveys the idea of something happening so quickly that it seems to occur in just one beat of the heart.
Sentences with «in a heartbeat»
I would go back again in a heartbeat if I was in the market for a new car. (autolist.com)
I am grateful to have had the opportunity to have been his student and would do it again in a heartbeat if life would allow me to. (samatva-yogalaya.com)
Love the color and print and if it had sleeves I'd buy in a heartbeat. (skirttherulesblog.com)