Check if character is number?

Question

I need to check whether justPrices[i].substr(commapos+2,1).

The string is something like: \"blabla,120\"

In this case it would check whether \'0

Answer 1

I think it's very fun to come up with ways to solve this. Below are some.
(All functions below assume argument is a single character. Change to n[0] to enforce it)

Method 1:

function isCharDigit(n){
  return !!n.trim() && n > -1;
}

Method 2:

function isCharDigit(n){
  return !!n.trim() && n*0==0;
}

Method 3:

function isCharDigit(n){
  return !!n.trim() && !!Number(n+.1); // "+.1' to make it work with "." and "0" Chars
}

Method 4:

var isCharDigit = (function(){
  var a = [1,1,1,1,1,1,1,1,1,1];
  return function(n){
    return !!a[n] // check if `a` Array has anything in index 'n'. Cast result to boolean
  }
})();

Method 5:

function isCharDigit(n){
  return !!n.trim() && !isNaN(+n);
}

Test string:

var str = ' 90ABcd#?:.+', char;
for( char of str ) 
  console.log( char, isCharDigit(char) );

Answer 2

Simple function

function isCharNumber(c){
    return c >= '0' && c <= '9';
}

Answer 3

This seems to work:

Static binding:

String.isNumeric = function (value) {
    return !isNaN(String(value) * 1);
};

Prototype binding:

String.prototype.isNumeric = function () {
    return !isNaN(this.valueOf() * 1);
};

It will check single characters, as well as whole strings to see if they are numeric.

Answer 4

Just use isFinite

const number = "1";
if (isFinite(number)) {
    // do something
}

Answer 5

If you are testing single characters, then:

var isDigit = (function() {
    var re = /^\d$/;
    return function(c) {
        return re.test(c);
    }
}());

will return true or false depending on whether c is a digit or not.

Answer 6

A simple solution by leveraging language's dynamic type checking:

function isNumber (string) {
   //it has whitespace
   if(string.trim() === ''){
     return false
   }
   return string - 0 === string * 1
}

see test cases below

function isNumber (str) {
   if(str.trim() === ''){
     return false
   }
   return str - 0 === str * 1
}


console.log('-1' + ' → ' + isNumber ('-1'))    
console.log('-1.5' + ' → ' + isNumber ('-1.5')) 
console.log('0' + ' → ' + isNumber ('0'))    
console.log(', ,' + ' → ' + isNumber (', ,'))  
console.log('0.42' + ' → ' + isNumber ('0.42'))   
console.log('.42' + ' → ' + isNumber ('.42'))    
console.log('#abcdef' + ' → ' + isNumber ('#abcdef'))
console.log('1.2.3' + ' → ' + isNumber ('1.2.3')) 
console.log('' + ' → ' + isNumber (''))    
console.log('blah' + ' → ' + isNumber ('blah'))