Returning Values in Arrow Functions

It's common knowledge that if a single statement follows the arrow function declaration, it returns the result of that declaration:

a=>{return a+3}
a=>a+3

-7 bytes

So when possible, combine multiple statements into one. This is most easily done by surrounding the statements with brackets and separating them with commas:

a=>{r=0;a.map(n=>r+=n);return r}
a=>(r=0,a.map(n=>r+=n),r)

-8 bytes

But if there are only two statements, it is usually possible (and shorter) to combine them with && or ||:

a=>{r=0;a.map(n=>r+=n);return r}

// - Use && because map always returns an array (true)
// - declaration of r moved into unused map argument to make it only 2 statements
a=>a.map(n=>r+=n,r=0)&&r

-9 bytes

Finally if you are using map (or similar) and need to return a number and you can guarantee the map will never return a 1-length array with a number, you can return the number with |:

a=>{a=b=0;a.map(n=>(a+=n,b-=n));return a/b}

// - {} in map ensures it returns an array of undefined, so the | will make the returned
//   array cast from [ undefined, undefined, undefined ] to ",," to NaN to 0 and 0|n = n,
//   if the map returned [ 4 ] it would cast from [ 4 ] to "4" to 4 and make it 4|n
a=>a.map(n=>{a+=n,b-=n},a=b=0)|a/b