function isNumberStr(str){    var n = Number(str);    return !isNaN(n);}

console.log(isNumberStr('37')); //true

console.log(isNumberStr('37.454')); //true

console.log(isNumberStr('37.454A')); //false

console.log(isNumberStr('0')); //true

console.log(isNumberStr('-37.454')); //true