const elegir = ( fn ) => {
  switch(fn) {
    case 'square':
      return (n) => n * n;
    case 'sum':
      return (n, m) => n + m;
    case 'mult':
      return (n, m) => n * m;
  }
}

const square = elegir('square');
const sum    = elegir('sum');
const mult   = elegir('mult');

console.log(square(5));
console.log(sum(1, 22));
console.log(mult(2, 8));