1.

return arr.reduce(function(x,y){return x*y;});

2.

var arr = s.split();

return arr.map(function (x){return +x;}).reduce(function (x,y){return x*10+y;});

3.

return arr.map(function (x){var tail = x.substring(1).toLowerCase();var head = x[0].toUpperCase();return head + tail;});

4.

r = arr.map(function (x){return +x;});