isNaN() vs typecast(varString).tostring() = varString

Question

After some string manipulation I am left with a 4 character string that I will call

varString

I need to see if this string could be interpreted as a number. I have been using

if(isNaN(Number(varString)) ==  false){return true}

but I am wondering if

if(Number("varString").toString() == varString){return true}

would be better.

Is one of these solutions more efficient, or more likely to catch non-numbers than the other? I have not found a difference while testing but I may have missed something. Alternative solutions welcome.

Edit in response to possible duplicate: I already have two ways of determining if something is a string or number. I am asking which method is better and why.

Answer 1

The second method will prove wrong for some cases, since Number will convert numbers starting with 0, but will save them in the form of a number, so a toString will not match the original string:

document.write(Number("0123").toString() == "0123")

---

Go with

return !isNaN(varString)

(a variation of the first one, and you dont need to parse with Number).

Answer 2

There will be differences at least in these cases:

• With "NaN"

  isNaN(Number("NaN")) === false; // false
  Number("NaN").toString() === "NaN"; // true
  

• With "-0"

  isNaN(Number("-0")) === false; // true
  Number("-0").toString() === "-0"; // false
  

• With space padded numbers

  isNaN(Number(" 1 ")) === false; // true
  Number(" 1 ").toString() === " 1 "; // false
  

• With zero left-padded numbers

  isNaN(Number("01")) === false; // true
  Number("01").toString() === "01"; // false
  

• With numbers starting or ending with decimal point

  isNaN(Number(".5")) === false; // true
  Number(".5").toString() === ".5"; // false
  

• With huge or tiny numbers

  isNaN(Number("999999999999999999999")) === false; // true
  Number("999999999999999999999").toString() === "999999999999999999999"; // false
  

• With non-huge and non-tiny numbers in exponential form

  isNaN(Number("1e1")) === false; // true
  Number("1e1").toString() === "1e1"; // false
  

• With binary, octal or hex literals

  isNaN(Number("0xa")) === false; // true
  Number("0xa").toString() === "0xa"; // false
  

Answer 3

In the second case you are parsing the number from string and converting it back to string. Obviously, it is less efficient