Question

MY code is:

function isNumber(n){
return typeof n == 'number' && !isNaN(n);
}

window.onload=function(){
var a=0,b=1,c=2.2,d=-3,e=-4.4,f=10/3;
var shouldBeTrue=[a,b,c,d,e,f];

var aa="0",bb="1",cc="2.2",dd="-3",ee="-4.4",ff="10/3";
var shouldBeFalse=[aa,bb,cc,dd,ee,ff];

var aaa,bbb=true,ccc=false,ddd=document.getElementsByTagName('html');
var alsoTheseBeFalse=[aaa,bbb,ccc,ddd,""," ",,null,NaN];

for(var i=0;i<shouldBeTrue.length;i++)
    if(isNumber(shouldBeTrue[i]) != true) alert("x");
for(i=0;i<shouldBeFalse.length;i++)
    if(isNumber(shouldBeFalse[i]) != false) alert("x");
for(i=0;i<alsoTheseBeFalse.length;i++)
    if(isNumber(alsoTheseBeFalse[i]) != false) alert("x");
}

What else should I check against to ensure my function is 101% perfect in all ways? (also, if you know a better function please tell me)

Answer 1

If you want to check whether a number is a real number, you should also check whether it’s finite:

function isNumber(n){
    return typeof n == 'number' && !isNaN(n) && isFinite(n);
 }

Another method (explanation below):

function isNumber(n){
    return typeof n == 'number' && !isNaN(n - n);
}

Update: Two expressions to validate a real number

Since JavaScript numbers are representing real numbers, the substraction operand on the same number should produce the zero value (additive identity). Numbers out of range should (and will) be invalid, NaN.

1        - 1        = 0    // OK
Infinity - Infinity = NaN  // Expected
NaN      - NaN      = NaN  // Expected
NaN      - Infinity = NaN