Number.prototype.toExponential()

function expo(x, f) {
  return Number.parseFloat(x).toExponential(f);
}

console.log(expo(123456, 2));
// : "1.23e+5"

console.log(expo("123456"));
// : "1.23456e+5"

console.log(expo("oink"));
// : "NaN"

toExponential()
toExponential(fractionDigits)

const numObj = 77.1234;

console.log(numObj.toExponential()); // 7.71234e+1 
console.log(numObj.toExponential(4)); // 7.7123e+1 
console.log(numObj.toExponential(2)); // 7.71e+1 
console.log((77.1234).toExponential()); // 7.71234e+1 
console.log((77).toExponential()); // 7.7e+1