I got a solution of 39 characters and after seeing yours i got to an even shorter one
f=(a,b)=>a.push(...b)
My first solution was
f=(a,b)=>[...a,...b].reduce((d)=>1+d,0)
Graduated in Digital Media M.Sc. now developing the next generation of educational software. Since a while I develop full stack in Javascript using Meteor. Love fitness and Muay Thai after work.
I got a solution of 39 characters and after seeing yours i got to an even shorter one
f=(a,b)=>a.push(...b)
My first solution was
f=(a,b)=>[...a,...b].reduce((d)=>1+d,0)
Impressing! So it's 21 characters at the lowest :-)