Math.pow():次方根

Math.pow():將其參數強制為數字。不可強制值將變為NaN，Math.abs()也使得返回NaN

Math.pow(x,y)=Xy
x:基數
y:指數

console.log(Math.pow(7, 3));
// Expected output: 343

console.log(Math.pow(4, 0.5));
// Expected output: 2

console.log(Math.pow(7, -2));
// Expected output: 0.02040816326530612
//                  (1/49)

console.log(Math.pow(-7, 0.5));
// Expected output: NaN