Why isNaN('') or isNaN(“”) is false (one quote or double quotes are considered a valid number)?

Question

I need a built in function that checks if a variable contains a valid number in Javascript , following this link I\'ve tried to use is isNaN , however when I use on

Answer 1

Converting an empty string to Number will evaluate to 0. Same goes for booleans (+false = 0, +true = 1), and null. If that's unwanted, you can create your own function to determine if some (string) value can be converted to Number. See also (the examples @) MDN.

const canConvert2Number = value =>
      !value ||
      value === false ||
      value === true ||
      value === null || 
      String(value).trim().length < 1
      ? false
      : !isNaN(+value);

console.log(canConvert2Number(null));    //false
console.log(canConvert2Number(""));      //false
console.log(canConvert2Number());        //false
console.log(canConvert2Number(false));   //false
console.log(canConvert2Number(true));    //false
console.log(canConvert2Number("[]"));    //false
console.log(canConvert2Number("{}"));    //false
console.log(canConvert2Number("20.4"));  //true
console.log(canConvert2Number("10E4"));  //true

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