How to check if a JavaScript number is a real, valid number?

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)

Solution 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

Solution 2

JS numbers can be among the following values:

• Finite numbers
• +Infinity and -Infinity
• NaN

Then there also non-number values which are coercible to numbers, e.g. number objects. You might want to consider them numerical.

If you only want to test finite numbers, simply use Number.isFinite:

Number.isFinite(value)

var isNumber = Number.isFinite;
assert('isNumber(1)', true);
assert('isNumber(1.1)', true);
assert('isNumber(+0)', true);
assert('isNumber(-0)', true);
assert('isNumber(-1.1)', true);
assert('isNumber(Math.PI)', true);
assert('isNumber(1e300)', true);
assert('isNumber(+Infinity)', false);
assert('isNumber(-Infinity)', false);
assert('isNumber(NaN)', false);
assert('isNumber(null)', false);
assert('isNumber(undefined)', false);
assert('isNumber(true)', false);
assert('isNumber(false)', false);
assert('isNumber("123")', false);
assert('isNumber("foo")', false);
assert('isNumber(new Number(1))', false);
assert('isNumber([])', false);
assert('isNumber({})', false);
assert('isNumber(function(){})', false);
function assert(code, expected) {
  var result = eval(code);
  console.log('Test ' + (result===expected ? 'pass' : 'FAIL') + ': ', code, ' -> ', result);
}

If you want to include infinities, check the type and exclude NaN:

typeof value === "number" && !Number.isNaN(value)

function isNumber(value) {
  return typeof value === "number" && !Number.isNaN(value);
}
assert('isNumber(1)', true);
assert('isNumber(1.1)', true);
assert('isNumber(+0)', true);
assert('isNumber(-0)', true);
assert('isNumber(-1.1)', true);
assert('isNumber(Math.PI)', true);
assert('isNumber(1e300)', true);
assert('isNumber(+Infinity)', true);
assert('isNumber(-Infinity)', true);
assert('isNumber(NaN)', false);
assert('isNumber(null)', false);
assert('isNumber(undefined)', false);
assert('isNumber(true)', false);
assert('isNumber(false)', false);
assert('isNumber("123")', false);
assert('isNumber("foo")', false);
assert('isNumber(new Number(1))', false);
assert('isNumber([])', false);
assert('isNumber({})', false);
assert('isNumber(function(){})', false);
function assert(code, expected) {
  var result = eval(code);
  console.log('Test ' + (result===expected ? 'pass' : 'FAIL') + ': ', code, ' -> ', result);
}

If you want to consider number objects as numbers, you can unwrap them using

value = Number.valueOf.call(value); // throws if value was not a number object

function isNumber(value) {
  try { value = Number.prototype.valueOf.call(value); } catch(err) { }
  return Number.isFinite(value);
}
assert('isNumber(1)', true);
assert('isNumber(1.1)', true);
assert('isNumber(+0)', true);
assert('isNumber(-0)', true);
assert('isNumber(-1.1)', true);
assert('isNumber(Math.PI)', true);
assert('isNumber(1e300)', true);
assert('isNumber(+Infinity)', false);
assert('isNumber(-Infinity)', false);
assert('isNumber(NaN)', false);
assert('isNumber(null)', false);
assert('isNumber(undefined)', false);
assert('isNumber(true)', false);
assert('isNumber(false)', false);
assert('isNumber("123")', false);
assert('isNumber("foo")', false);
assert('isNumber(new Number(1))', true);
assert('isNumber([])', false);
assert('isNumber({})', false);
assert('isNumber(function(){})', false);
function assert(code, expected) {
  var result = eval(code);
  console.log('Test ' + (result===expected ? 'pass' : 'FAIL') + ': ', code, ' -> ', result);
}

If you want to include arbitrary values coercible to numbers, you can use the unary + to coerce.

value = +value; // throws if value was not number-coercible

There is also the isNaN function (not to be confused with Number.isNaN), which will first coerce and then compare with NaN. But be aware whitespace strings and null are coerced to +0, not NaN. So you might be interested in Validate decimal numbers in JavaScript - IsNumeric()

Comments

[frenchie]

which function are you looking to make perfect? You could start by giving your variables more meaningful names...

[Rob W]

Side note, inside your test cases, you're using != false. This is not needed, because your isNumber function always return a boolean (true or false). Within if ( ... ), an expressions is always treathed as a boolean. if (isNumber(a) != false) is equivalent to if (isNumber(a) == true) is equivalent to if (isNumber(a)).

[Admin]

Well, your code is larger and do 1 more operation. The only way I would use it is if you to tell me a way that my current function would fail and yours not

[Rob W]

@TuxedoKnightChess Input the following value: Infinity. It passes the first two tests, while it's not a useful real number: isNumber(Infinity) returns true for your current function.

[Admin]

what would be the result for var n=2E30;?

[RightSaidFred]

@TuxedoKnightChess: There's a real easy way to find out.

[Rob W]

@TuxedoKnightChess I've updated the answer with another method. Two tests instead of one.

[Admin]

Just to double check, both of your functions do exactly the same thing?

[Rob W]

@TuxedoKnightChess The methods slightly differ, but the results are exactely equal.

[RightSaidFred]

@down-voter: Give a reason. Please tell me how new Number(123) will pass the isNumber test in the question.

[Rob W]

I did not downvote, but your code only shows an example of something that breaks the code, without proposing a solution. The OP did not ask how to break the function, but how to improve the function.

[Dmitri Zaitsev]

Real numbers are finite per definition.

[João Pimentel Ferreira]

how come this function is not embedded in Javascript, is for me a big huge surprise. Dealing with true numbers (integers or float) is a basic task of any programming language

[João Pimentel Ferreira]

btw, can I give 10 of my points to you. I feel like +1 is just not enough

[João Pimentel Ferreira]

your isNumber(n) function is perfect . No variable coercion and all that typical mess of javascript, no string, no "1", no true, no "true", no NaN, no null, no 1/0. n must simply be a finite valid number (integer or float). And 0 returns true as it should be.

[João Pimentel Ferreira]

@DmitriZaitsev what the OP meant is not real in the mathematical sense, but merely a true number (natural, integer, rational or real numbers, in the mathematical sense). Javascript is a mess dealing with types and type coercion and believe me that this function is very, very important.