In a heartbeat
Definition 1 of 1
Pro Tip 1/3Emotional Intensity
Used to express a strong emotional response or desire to do something without delay. 
If she called me for help, I'd be there in a heartbeat.
Pro Tip 2/3Informal Use
It's casual and mostly used in spoken English or informal writing, like text messages or emails between friends. 
I'd join your band in a heartbeat, just ask!
Pro Tip 3/3Positive Scenarios
Often used when talking about an opportunity or an action someone highly favors or is excited about. 
He'd adopt another puppy in a heartbeat if he could.