How to test if a string is a not number - javascript

I'm trying to verify if a string is text or number.
Could not find a proper way to verify.
Could you please advice?
Here is my problem:
var myNumber = "006";
var myText = "1. This is not a number";
isNaN(myNumber); // false
isNaN(myText); // false
I tried also:
isNaN(myNumber.split('.')[1]); // true
isNaN(myText.split('.')[1]); // true
parseInt(myNumber); // 6
parseInt(myText); // 1
What I would like to achieve would be to find when a string can be converted to a number (see myNumber). In case that the string is actually a text, how to spot it with javascript?
Could you please advise?

If I understood your question correctly I may have a solution but this will work even if the string is a number so here you go:
var yourNumber="55";
if(yourNumber*1==yourNumber){
alert("It's a number");
}else{alert("it's not a number");}

If parseInt() is not working for your desired results, you can try Number constructor. It gives you NaN for non-numbers, which you can verify by using isNaN() function.
var myNumber = "006";
var myText = "1. This is not a number";
console.log( Number(myNumber) );
console.log( Number(myText) );
Or you can use regular expressions:
var myNumber = "006";
var myText = "1. This is not a number";
var numRegex = /^\d+$/;
console.log( numRegex.test(myNumber) );
console.log( numRegex.test(myText) );

You can use regex.
function isNumber(num){
return /^(\d+)?(\.)?\d+$/.test(num);
}
isNumber("006") // true
isNumber(".6") // true
isNumber("1 not a number") // false
isNumber("23.63") // true
isNumber("23.6.3") // false

You can check if text is a number using isNaN function:
var myNumber = "006";
var myText = "1. This is not a number";
console.log(myNumber + ': ' + !isNaN(myNumber));
console.log(myText + ': ' + !isNaN(myText));

var reg = new RegExp('^[0-9]*$');
var myNumber = "006";
var myText = "1. This is not a number";
reg.test(myNumber) //true
reg.test(myText) //false

The short story:
I would really use the typeof operator.
From isNaN() | JavaScript MDN:
The isNaN() function determines whether a value is Not-A-Number or not. ... you may alternatively want to use Number.isNaN(), as defined in ECMAScript 6, or you can use typeof to determine if the value is Not-A-Number.
Long story:
Tried the following in a node console.
A NaN also results from attempted coercion to numeric values of non-numeric values for which no primitive numeric value is available.
isNaN(123) // false
isNaN(true) // false
isNaN("123") // false
isNaN({}) // true
isNaN(undefined) // true
In comparison to the global isNaN() function, Number.isNaN() doesn't suffer the problem of forcefully converting the parameter to a number. This means it is now safe to pass values that would normally convert to NaN, but aren't actually the same value as NaN. This also means that only values of the type number, that are also NaN, return true.
Number.isNaN(undefined) // false
Number.isNaN({}) // false
Number.isNaN(true) // false
Number.isNaN(123) // false
Number.isNaN(NaN) // true
Number.isNaN(0/0) // true
The typeof operator returns a string indicating the type of the unevaluated operand.
typeof(123) // number
typeof("123") // string
typeof(true) // boolean

Generally, the isNaN is the right idea, but you should parse it first:
var myNumber = "006";
var myText = "1. This is not a number";
Number.isNaN(Number.parseInt(myNumber)); // false
Number.isNaN(Number.parseInt(myText)); // true

Related

Something strange happens with parseInt() [duplicate]

This question already has answers here:
What is the rationale for all comparisons returning false for IEEE754 NaN values?
(12 answers)
Closed 1 year ago.
So, I have the following issue:
let str = "M1"
console.log(parseInt(str.charAt(0)) != NaN))
It says true for some reason. Same with 1M.
As long as I know, NaN is a number type.
Though, I need (parseInt(str.charAt(0)) != NaN) == false in this case. And true in case of 1M.
Thanks for every answer.
PS* parseInt(str) returns NaN
Rather than trying to prove a negative and prove that something is not NaN (remembering that NaN istelf means Not a Number) and expecting a false... too many double negtives
try simply isNaN(str[0])
let str1 = "M1";
let str2 = "1M";
let result1 = isNaN(str1[0]);
let result2 = isNaN(str2[0]);
console.log(result1); // gives true - ie: str1[0] is not a number
console.log(result2); // gives false -ie: str2[0] a number
NaN === NaN is false, so you can't just compare them with === or !==
You can use isNaN(parseInt(str.charAt(0)) instead.
You can't compare NaN with == operator. Easily check it in if:
let str = "M1"
if(!parseInt(str.charAt(0)))
console.log("NaN")
It is working as expected. NaN is global property and is of not type number.
Here parseInt('M1'.charAt(0)) != NaN will be tested as NaN !== NaN. Since NaN is a global object these two NaN are not pointing same object but two different objects. So it is returning true
In second case (parseInt('1M'.charAt(0)) !== NaN), it is obvious true as 1 !== NaN.
Note: Use === instead of ==
Because: NaN is not equal to NaN
let str = "M1"
console.log(parseInt(str.charAt(0)) != NaN))
parseInt(str.charAt(0)) is a NaN value which is not equal to another NaN value.
Check this out NaN - JavaScript | MDN

Javascript: NaN vs typeof Number [duplicate]

I'm hoping there's something in the same conceptual space as the old VB6 IsNumeric() function?
2nd October 2020: note that many bare-bones approaches are fraught with subtle bugs (eg. whitespace, implicit partial parsing, radix, coercion of arrays etc.) that many of the answers here fail to take into account. The following implementation might work for you, but note that it does not cater for number separators other than the decimal point ".":
function isNumeric(str) {
if (typeof str != "string") return false // we only process strings!
return !isNaN(str) && // use type coercion to parse the _entirety_ of the string (`parseFloat` alone does not do this)...
!isNaN(parseFloat(str)) // ...and ensure strings of whitespace fail
}
To check if a variable (including a string) is a number, check if it is not a number:
This works regardless of whether the variable content is a string or number.
isNaN(num) // returns true if the variable does NOT contain a valid number
Examples
isNaN(123) // false
isNaN('123') // false
isNaN('1e10000') // false (This translates to Infinity, which is a number)
isNaN('foo') // true
isNaN('10px') // true
isNaN('') // false
isNaN(' ') // false
isNaN(false) // false
Of course, you can negate this if you need to. For example, to implement the IsNumeric example you gave:
function isNumeric(num){
return !isNaN(num)
}
To convert a string containing a number into a number:
Only works if the string only contains numeric characters, else it returns NaN.
+num // returns the numeric value of the string, or NaN
// if the string isn't purely numeric characters
Examples
+'12' // 12
+'12.' // 12
+'12..' // NaN
+'.12' // 0.12
+'..12' // NaN
+'foo' // NaN
+'12px' // NaN
To convert a string loosely to a number
Useful for converting '12px' to 12, for example:
parseInt(num) // extracts a numeric value from the
// start of the string, or NaN.
Examples
parseInt('12') // 12
parseInt('aaa') // NaN
parseInt('12px') // 12
parseInt('foo2') // NaN These last three may
parseInt('12a5') // 12 be different from what
parseInt('0x10') // 16 you expected to see.
Floats
Bear in mind that, unlike +num, parseInt (as the name suggests) will convert a float into an integer by chopping off everything following the decimal point (if you want to use parseInt() because of this behaviour, you're probably better off using another method instead):
+'12.345' // 12.345
parseInt(12.345) // 12
parseInt('12.345') // 12
Empty strings
Empty strings may be a little counter-intuitive. +num converts empty strings or strings with spaces to zero, and isNaN() assumes the same:
+'' // 0
+' ' // 0
isNaN('') // false
isNaN(' ') // false
But parseInt() does not agree:
parseInt('') // NaN
parseInt(' ') // NaN
If you're just trying to check if a string is a whole number (no decimal places), regex is a good way to go. Other methods such as isNaN are too complicated for something so simple.
function isNumeric(value) {
return /^-?\d+$/.test(value);
}
console.log(isNumeric('abcd')); // false
console.log(isNumeric('123a')); // false
console.log(isNumeric('1')); // true
console.log(isNumeric('1234567890')); // true
console.log(isNumeric('-23')); // true
console.log(isNumeric(1234)); // true
console.log(isNumeric(1234n)); // true
console.log(isNumeric('123.4')); // false
console.log(isNumeric('')); // false
console.log(isNumeric(undefined)); // false
console.log(isNumeric(null)); // false
To only allow positive whole numbers use this:
function isNumeric(value) {
return /^\d+$/.test(value);
}
console.log(isNumeric('123')); // true
console.log(isNumeric('-23')); // false
The accepted answer for this question has quite a few flaws (as highlighted by couple of other users). This is one of the easiest & proven way to approach it in javascript:
function isNumeric(n) {
return !isNaN(parseFloat(n)) && isFinite(n);
}
Below are some good test cases:
console.log(isNumeric(12345678912345678912)); // true
console.log(isNumeric('2 ')); // true
console.log(isNumeric('-32.2 ')); // true
console.log(isNumeric(-32.2)); // true
console.log(isNumeric(undefined)); // false
// the accepted answer fails at these tests:
console.log(isNumeric('')); // false
console.log(isNumeric(null)); // false
console.log(isNumeric([])); // false
And you could go the RegExp-way:
var num = "987238";
if(num.match(/^-?\d+$/)){
//valid integer (positive or negative)
}else if(num.match(/^\d+\.\d+$/)){
//valid float
}else{
//not valid number
}
If you really want to make sure that a string contains only a number, any number (integer or floating point), and exactly a number, you cannot use parseInt()/ parseFloat(), Number(), or !isNaN() by themselves. Note that !isNaN() is actually returning true when Number() would return a number, and false when it would return NaN, so I will exclude it from the rest of the discussion.
The problem with parseFloat() is that it will return a number if the string contains any number, even if the string doesn't contain only and exactly a number:
parseFloat("2016-12-31") // returns 2016
parseFloat("1-1") // return 1
parseFloat("1.2.3") // returns 1.2
The problem with Number() is that it will return a number in cases where the passed value is not a number at all!
Number("") // returns 0
Number(" ") // returns 0
Number(" \u00A0 \t\n\r") // returns 0
The problem with rolling your own regex is that unless you create the exact regex for matching a floating point number as Javascript recognizes it you are going to miss cases or recognize cases where you shouldn't. And even if you can roll your own regex, why? There are simpler built-in ways to do it.
However, it turns out that Number() (and isNaN()) does the right thing for every case where parseFloat() returns a number when it shouldn't, and vice versa. So to find out if a string is really exactly and only a number, call both functions and see if they both return true:
function isNumber(str) {
if (typeof str != "string") return false // we only process strings!
// could also coerce to string: str = ""+str
return !isNaN(str) && !isNaN(parseFloat(str))
}
Try the isNan function:
The isNaN() function determines whether a value is an illegal number (Not-a-Number).
This function returns true if the value equates to NaN. Otherwise it returns false.
This function is different from the Number specific Number.isNaN() method.
The global isNaN() function, converts the tested value to a Number, then tests it.
Number.isNan() does not convert the values to a Number, and will not return true for any value that is not of the type Number...
2019: Including ES3, ES6 and TypeScript Examples
Maybe this has been rehashed too many times, however I fought with this one today too and wanted to post my answer, as I didn't see any other answer that does it as simply or thoroughly:
ES3
var isNumeric = function(num){
return (typeof(num) === 'number' || typeof(num) === "string" && num.trim() !== '') && !isNaN(num);
}
ES6
const isNumeric = (num) => (typeof(num) === 'number' || typeof(num) === "string" && num.trim() !== '') && !isNaN(num);
Typescript
const isNumeric = (num: any) => (typeof(num) === 'number' || typeof(num) === "string" && num.trim() !== '') && !isNaN(num as number);
This seems quite simple and covers all the bases I saw on the many other posts and thought up myself:
// Positive Cases
console.log(0, isNumeric(0) === true);
console.log(1, isNumeric(1) === true);
console.log(1234567890, isNumeric(1234567890) === true);
console.log('1234567890', isNumeric('1234567890') === true);
console.log('0', isNumeric('0') === true);
console.log('1', isNumeric('1') === true);
console.log('1.1', isNumeric('1.1') === true);
console.log('-1', isNumeric('-1') === true);
console.log('-1.2354', isNumeric('-1.2354') === true);
console.log('-1234567890', isNumeric('-1234567890') === true);
console.log(-1, isNumeric(-1) === true);
console.log(-32.1, isNumeric(-32.1) === true);
console.log('0x1', isNumeric('0x1') === true); // Valid number in hex
// Negative Cases
console.log(true, isNumeric(true) === false);
console.log(false, isNumeric(false) === false);
console.log('1..1', isNumeric('1..1') === false);
console.log('1,1', isNumeric('1,1') === false);
console.log('-32.1.12', isNumeric('-32.1.12') === false);
console.log('[blank]', isNumeric('') === false);
console.log('[spaces]', isNumeric(' ') === false);
console.log('null', isNumeric(null) === false);
console.log('undefined', isNumeric(undefined) === false);
console.log([], isNumeric([]) === false);
console.log('NaN', isNumeric(NaN) === false);
You can also try your own isNumeric function and just past in these use cases and scan for "true" for all of them.
Or, to see the values that each return:
Old question, but there are several points missing in the given answers.
Scientific notation.
!isNaN('1e+30') is true, however in most of the cases when people ask for numbers, they do not want to match things like 1e+30.
Large floating numbers may behave weird
Observe (using Node.js):
> var s = Array(16 + 1).join('9')
undefined
> s.length
16
> s
'9999999999999999'
> !isNaN(s)
true
> Number(s)
10000000000000000
> String(Number(s)) === s
false
>
On the other hand:
> var s = Array(16 + 1).join('1')
undefined
> String(Number(s)) === s
true
> var s = Array(15 + 1).join('9')
undefined
> String(Number(s)) === s
true
>
So, if one expects String(Number(s)) === s, then better limit your strings to 15 digits at most (after omitting leading zeros).
Infinity
> typeof Infinity
'number'
> !isNaN('Infinity')
true
> isFinite('Infinity')
false
>
Given all that, checking that the given string is a number satisfying all of the following:
non scientific notation
predictable conversion to Number and back to String
finite
is not such an easy task. Here is a simple version:
function isNonScientificNumberString(o) {
if (!o || typeof o !== 'string') {
// Should not be given anything but strings.
return false;
}
return o.length <= 15 && o.indexOf('e+') < 0 && o.indexOf('E+') < 0 && !isNaN(o) && isFinite(o);
}
However, even this one is far from complete. Leading zeros are not handled here, but they do screw the length test.
I have tested and Michael's solution is best. Vote for his answer above (search this page for "If you really want to make sure that a string" to find it). In essence, his answer is this:
function isNumeric(num){
num = "" + num; //coerce num to be a string
return !isNaN(num) && !isNaN(parseFloat(num));
}
It works for every test case, which I documented here:
https://jsfiddle.net/wggehvp9/5/
Many of the other solutions fail for these edge cases:
' ', null, "", true, and [].
In theory, you could use them, with proper error handling, for example:
return !isNaN(num);
or
return (+num === +num);
with special handling for
/\s/, null, "", true, false, [] (and others?)
TL;DR
It depends largely on what you want to parse as a number.
Comparison Between Built-in Functions
As none of the existing sources satisfied my soul, I tried to figure out what actually was happening with these functions.
Three immediate answers to this question felt like:
!isNaN(input) (which gives the same output as +input === +input)
!isNaN(parseFloat(input))
isFinite(input)
But are any of them correct in every scenario?
I tested these functions in several cases, and generated output as markdown. This is what it looks like:
input
!isNaN(input) or +input===+input
!isNaN(parseFloat(input))
isFinite(input)
Comment
123
✔️
✔️
✔️
-
'123'
✔️
✔️
✔️
-
12.3
✔️
✔️
✔️
-
'12.3'
✔️
✔️
✔️
-
'   12.3   '
✔️
✔️
✔️
Empty whitespace trimmed, as expected.
1_000_000
✔️
✔️
✔️
Numeric separator understood, also expected.
'1_000_000'
❌
✔️
❌
Surprise! JS just won't parse numeric separator inside a string. For details, check this issue. (Why then parsing as float worked though? Well, it didn't. 😉)
'0b11111111'
✔️
✔️
✔️
Binary form understood, as it should've.
'0o377'
✔️
✔️
✔️
Octal form understood too.
'0xFF'
✔️
✔️
✔️
Of course hex is understood. Did anybody think otherwise? 😒
''
✔️
❌
✔️
Should empty string be a number?
'    '
✔️
❌
✔️
Should a whitespace-only string be a number?
'abc'
❌
❌
❌
Everybody agrees, not a number.
'12.34Ab!##$'
❌
✔️
❌
Ah! Now it's quite understandable what parseFloat() does. Not impressive to me, but may come handy in certain cases.
'10e100'
✔️
✔️
✔️
10100 is a number indeed. But caution! It's way more larger than the maximum safe integer value 253 (about 9×1015). Read this for details.
'10e1000'
✔️
✔️
❌
Say with me, help! Though not as crazy as it may seem. In JavaScript, a value larger than ~10308 is rounded to infinity, that's why. Look here for details. And yes, isNaN() considers infinity as a number, and parseFloat() parses infinity as infinity.
null
✔️
❌
✔️
Now this is awkward. In JS, when a conversion is needed, null becomes zero, and we get a finite number. Then why parseFloat(null) should return a NaN here? Someone please explain this design concept to me.
undefined
❌
❌
❌
As expected.
Infinity
✔️
✔️
❌
As explained before, isNaN() considers infinity as a number, and parseFloat() parses infinity as infinity.
So...which of them is "correct"?
Should be clear by now, it depends largely on what we need. For example, we may want to consider a null input as 0. In that case isFinite() will work fine.
Again, perhaps we will take a little help from isNaN() when 1010000000000 is needed to be considered a valid number (although the question remains—why would it be, and how would we handle that)!
Of course, we can manually exclude any of the scenarios.
Like in my case, I needed exactly the outputs of isFinite(), except for the null case, the empty string case, and the whitespace-only string case. Also I had no headache about really huge numbers. So my code looked like this:
/**
* My necessity was met by the following code.
*/
if (input === null) {
// Null input
} else if (input.trim() === '') {
// Empty or whitespace-only string
} else if (isFinite(input)) {
// Input is a number
} else {
// Not a number
}
And also, this was my JavaScript to generate the table:
/**
* Note: JavaScript does not print numeric separator inside a number.
* In that single case, the markdown output was manually corrected.
* Also, the comments were manually added later, of course.
*/
let inputs = [
123, '123', 12.3, '12.3', ' 12.3 ',
1_000_000, '1_000_000',
'0b11111111', '0o377', '0xFF',
'', ' ',
'abc', '12.34Ab!##$',
'10e100', '10e1000',
null, undefined, Infinity];
let markdownOutput = `| \`input\` | \`!isNaN(input)\` or <br>\`+input === +input\` | \`!isNaN(parseFloat(input))\` | \`isFinite(input)\` | Comment |
| :---: | :---: | :---: | :---: | :--- |\n`;
for (let input of inputs) {
let outputs = [];
outputs.push(!isNaN(input));
outputs.push(!isNaN(parseFloat(input)));
outputs.push(isFinite(input));
if (typeof input === 'string') {
// Output with quotations
console.log(`'${input}'`);
markdownOutput += `| '${input}'`;
} else {
// Output without quotes
console.log(input);
markdownOutput += `| ${input}`;
}
for (let output of outputs) {
console.log('\t' + output);
if (output === true) {
markdownOutput += ` | <div style="color:limegreen">true</div>`;
// markdownOutput += ` | ✔️`; // for stackoverflow
} else {
markdownOutput += ` | <div style="color:orangered">false</div>`;
// markdownOutput += ` | ❌`; // for stackoverflow
}
}
markdownOutput += ` ||\n`;
}
// Replace two or more whitespaces with $nbsp;
markdownOutput = markdownOutput.replaceAll(` `, ` `);
// Print markdown to console
console.log(markdownOutput);
The JavaScript global isFinite() checks if a value is a valid (finite) number.
See MDN for the difference between Number.isFinite() and global isFinite().
let a = isFinite('abc') // false
let b = isFinite('123') // true
let c = isFinite('12a') // false
let d = isFinite(null) // true
console.log(a, b, c, d)
Someone may also benefit from a regex based answer. Here it is:
One liner isInteger:
const isInteger = num => /^-?[0-9]+$/.test(num+'');
One liner isNumeric: Accepts integers and decimals
const isNumeric = num => /^-?[0-9]+(?:\.[0-9]+)?$/.test(num+'');
You can use the result of Number when passing an argument to its constructor.
If the argument (a string) cannot be converted into a number, it returns NaN, so you can determinate if the string provided was a valid number or not.
Notes: Note when passing empty string or '\t\t' and '\n\t' as Number will return 0; Passing true will return 1 and false returns 0.
Number('34.00') // 34
Number('-34') // -34
Number('123e5') // 12300000
Number('123e-5') // 0.00123
Number('999999999999') // 999999999999
Number('9999999999999999') // 10000000000000000 (integer accuracy up to 15 digit)
Number('0xFF') // 255
Number('Infinity') // Infinity
Number('34px') // NaN
Number('xyz') // NaN
Number('true') // NaN
Number('false') // NaN
// cavets
Number(' ') // 0
Number('\t\t') // 0
Number('\n\t') // 0
Maybe there are one or two people coming across this question who need a much stricter check than usual (like I did). In that case, this might be useful:
if(str === String(Number(str))) {
// it's a "perfectly formatted" number
}
Beware! This will reject strings like .1, 40.000, 080, 00.1. It's very picky - the string must match the "most minimal perfect form" of the number for this test to pass.
It uses the String and Number constructor to cast the string to a number and back again and thus checks if the JavaScript engine's "perfect minimal form" (the one it got converted to with the initial Number constructor) matches the original string.
Why is jQuery's implementation not good enough?
function isNumeric(a) {
var b = a && a.toString();
return !$.isArray(a) && b - parseFloat(b) + 1 >= 0;
};
Michael suggested something like this (although I've stolen "user1691651 - John"'s altered version here):
function isNumeric(num){
num = "" + num; //coerce num to be a string
return !isNaN(num) && !isNaN(parseFloat(num));
}
The following is a solution with most likely bad performance, but solid results. It is a contraption made from the jQuery 1.12.4 implementation and Michael's answer, with an extra check for leading/trailing spaces (because Michael's version returns true for numerics with leading/trailing spaces):
function isNumeric(a) {
var str = a + "";
var b = a && a.toString();
return !$.isArray(a) && b - parseFloat(b) + 1 >= 0 &&
!/^\s+|\s+$/g.test(str) &&
!isNaN(str) && !isNaN(parseFloat(str));
};
The latter version has two new variables, though. One could get around one of those, by doing:
function isNumeric(a) {
if ($.isArray(a)) return false;
var b = a && a.toString();
a = a + "";
return b - parseFloat(b) + 1 >= 0 &&
!/^\s+|\s+$/g.test(a) &&
!isNaN(a) && !isNaN(parseFloat(a));
};
I haven't tested any of these very much, by other means than manually testing the few use-cases I'll be hitting with my current predicament, which is all very standard stuff. This is a "standing-on-the-shoulders-of-giants" situation.
2019: Practical and tight numerical validity check
Often, a 'valid number' means a Javascript number excluding NaN and Infinity, ie a 'finite number'.
To check the numerical validity of a value (from an external source for example), you can define in ESlint Airbnb style :
/**
* Returns true if 'candidate' is a finite number or a string referring (not just 'including') a finite number
* To keep in mind:
* Number(true) = 1
* Number('') = 0
* Number(" 10 ") = 10
* !isNaN(true) = true
* parseFloat('10 a') = 10
*
* #param {?} candidate
* #return {boolean}
*/
function isReferringFiniteNumber(candidate) {
if (typeof (candidate) === 'number') return Number.isFinite(candidate);
if (typeof (candidate) === 'string') {
return (candidate.trim() !== '') && Number.isFinite(Number(candidate));
}
return false;
}
and use it this way:
if (isReferringFiniteNumber(theirValue)) {
myCheckedValue = Number(theirValue);
} else {
console.warn('The provided value doesn\'t refer to a finite number');
}
It is not valid for TypeScript as:
declare function isNaN(number: number): boolean;
For TypeScript you can use:
/^\d+$/.test(key)
I like the simplicity of this.
Number.isNaN(Number(value))
The above is regular Javascript, but I'm using this in conjunction with a typescript typeguard for smart type checking. This is very useful for the typescript compiler to give you correct intellisense, and no type errors.
Typescript typeguards
Warning: See Jeremy's comment below. This has some issues with certain values and I don't have time to fix it now, but the idea of using a typescript typeguard is useful so I won't delete this section.
isNotNumber(value: string | number): value is string {
return Number.isNaN(Number(this.smartImageWidth));
}
isNumber(value: string | number): value is number {
return Number.isNaN(Number(this.smartImageWidth)) === false;
}
Let's say you have a property width which is number | string. You may want to do logic based on whether or not it's a string.
var width: number|string;
width = "100vw";
if (isNotNumber(width))
{
// the compiler knows that width here must be a string
if (width.endsWith('vw'))
{
// we have a 'width' such as 100vw
}
}
else
{
// the compiler is smart and knows width here must be number
var doubleWidth = width * 2;
}
The typeguard is smart enough to constrain the type of width within the if statement to be ONLY string. This permits the compiler to allow width.endsWith(...) which it wouldn't allow if the type was string | number.
You can call the typeguard whatever you want isNotNumber, isNumber, isString, isNotString but I think isString is kind of ambiguous and harder to read.
parseInt(), but be aware that this function is a bit different in the sense that it for example returns 100 for parseInt("100px").
When guarding against empty strings and null
// Base cases that are handled properly
Number.isNaN(Number('1')); // => false
Number.isNaN(Number('-1')); // => false
Number.isNaN(Number('1.1')); // => false
Number.isNaN(Number('-1.1')); // => false
Number.isNaN(Number('asdf')); // => true
Number.isNaN(Number(undefined)); // => true
// Special notation cases that are handled properly
Number.isNaN(Number('1e1')); // => false
Number.isNaN(Number('1e-1')); // => false
Number.isNaN(Number('-1e1')); // => false
Number.isNaN(Number('-1e-1')); // => false
Number.isNaN(Number('0b1')); // => false
Number.isNaN(Number('0o1')); // => false
Number.isNaN(Number('0xa')); // => false
// Edge cases that will FAIL if not guarded against
Number.isNaN(Number('')); // => false
Number.isNaN(Number(' ')); // => false
Number.isNaN(Number(null)); // => false
// Edge cases that are debatable
Number.isNaN(Number('-0b1')); // => true
Number.isNaN(Number('-0o1')); // => true
Number.isNaN(Number('-0xa')); // => true
Number.isNaN(Number('Infinity')); // => false
Number.isNaN(Number('INFINITY')); // => true
Number.isNaN(Number('-Infinity')); // => false
Number.isNaN(Number('-INFINITY')); // => true
When NOT guarding against empty strings and null
Using parseInt:
// Base cases that are handled properly
Number.isNaN(parseInt('1')); // => false
Number.isNaN(parseInt('-1')); // => false
Number.isNaN(parseInt('1.1')); // => false
Number.isNaN(parseInt('-1.1')); // => false
Number.isNaN(parseInt('asdf')); // => true
Number.isNaN(parseInt(undefined)); // => true
Number.isNaN(parseInt('')); // => true
Number.isNaN(parseInt(' ')); // => true
Number.isNaN(parseInt(null)); // => true
// Special notation cases that are handled properly
Number.isNaN(parseInt('1e1')); // => false
Number.isNaN(parseInt('1e-1')); // => false
Number.isNaN(parseInt('-1e1')); // => false
Number.isNaN(parseInt('-1e-1')); // => false
Number.isNaN(parseInt('0b1')); // => false
Number.isNaN(parseInt('0o1')); // => false
Number.isNaN(parseInt('0xa')); // => false
// Edge cases that are debatable
Number.isNaN(parseInt('-0b1')); // => false
Number.isNaN(parseInt('-0o1')); // => false
Number.isNaN(parseInt('-0xa')); // => false
Number.isNaN(parseInt('Infinity')); // => true
Number.isNaN(parseInt('INFINITY')); // => true
Number.isNaN(parseInt('-Infinity')); // => true
Number.isNaN(parseInt('-INFINITY')); // => true
Using parseFloat:
// Base cases that are handled properly
Number.isNaN(parseFloat('1')); // => false
Number.isNaN(parseFloat('-1')); // => false
Number.isNaN(parseFloat('1.1')); // => false
Number.isNaN(parseFloat('-1.1')); // => false
Number.isNaN(parseFloat('asdf')); // => true
Number.isNaN(parseFloat(undefined)); // => true
Number.isNaN(parseFloat('')); // => true
Number.isNaN(parseFloat(' ')); // => true
Number.isNaN(parseFloat(null)); // => true
// Special notation cases that are handled properly
Number.isNaN(parseFloat('1e1')); // => false
Number.isNaN(parseFloat('1e-1')); // => false
Number.isNaN(parseFloat('-1e1')); // => false
Number.isNaN(parseFloat('-1e-1')); // => false
Number.isNaN(parseFloat('0b1')); // => false
Number.isNaN(parseFloat('0o1')); // => false
Number.isNaN(parseFloat('0xa')); // => false
// Edge cases that are debatable
Number.isNaN(parseFloat('-0b1')); // => false
Number.isNaN(parseFloat('-0o1')); // => false
Number.isNaN(parseFloat('-0xa')); // => false
Number.isNaN(parseFloat('Infinity')); // => false
Number.isNaN(parseFloat('INFINITY')); // => true
Number.isNaN(parseFloat('-Infinity')); // => false
Number.isNaN(parseFloat('-INFINITY')); // => true
Notes:
Only string, empty, and uninitialized values are considered in keeping with addressing the original question. Additional edge cases exist if arrays and objects are the values being considered.
Characters in binary, octal, hexadecimal, and exponential notation are not case-sensitive (ie: '0xFF', '0XFF', '0xfF' etc. will all yield the same result in the test cases shown above).
Unlike with Infinity (case-sensitive) in some cases, constants from the Number and Math objects passed as test cases in string format to any of the methods above will be determined to not be numbers.
See here for an explanation of how arguments are converted to a Number and why the edge cases for null and empty strings exist.
Quote:
isNaN(num) // returns true if the variable does NOT contain a valid number
is not entirely true if you need to check for leading/trailing spaces - for example when a certain quantity of digits is required, and you need to get, say, '1111' and not ' 111' or '111 ' for perhaps a PIN input.
Better to use:
var num = /^\d+$/.test(num)
This is built on some of the previous answers and comments. The following covers all the edge cases and fairly concise as well:
const isNumRegEx = /^-?(\d*\.)?\d+$/;
function isNumeric(n, allowScientificNotation = false) {
return allowScientificNotation ?
!Number.isNaN(parseFloat(n)) && Number.isFinite(n) :
isNumRegEx.test(n);
}
If anyone ever gets this far down, I spent some time hacking on this trying to patch moment.js (https://github.com/moment/moment). Here's something that I took away from it:
function isNumeric(val) {
var _val = +val;
return (val !== val + 1) //infinity check
&& (_val === +val) //Cute coercion check
&& (typeof val !== 'object') //Array/object check
}
Handles the following cases:
True! :
isNumeric("1"))
isNumeric(1e10))
isNumeric(1E10))
isNumeric(+"6e4"))
isNumeric("1.2222"))
isNumeric("-1.2222"))
isNumeric("-1.222200000000000000"))
isNumeric("1.222200000000000000"))
isNumeric(1))
isNumeric(0))
isNumeric(-0))
isNumeric(1010010293029))
isNumeric(1.100393830000))
isNumeric(Math.LN2))
isNumeric(Math.PI))
isNumeric(5e10))
False! :
isNumeric(NaN))
isNumeric(Infinity))
isNumeric(-Infinity))
isNumeric())
isNumeric(undefined))
isNumeric('[1,2,3]'))
isNumeric({a:1,b:2}))
isNumeric(null))
isNumeric([1]))
isNumeric(new Date()))
Ironically, the one I am struggling with the most:
isNumeric(new Number(1)) => false
Any suggestions welcome. :]
I recently wrote an article about ways to ensure a variable is a valid number: https://github.com/jehugaleahsa/artifacts/blob/master/2018/typescript_num_hack.md The article explains how to ensure floating point or integer, if that's important (+x vs ~~x).
The article assumes the variable is a string or a number to begin with and trim is available/polyfilled. It wouldn't be hard to extend it to handle other types, as well. Here's the meat of it:
// Check for a valid float
if (x == null
|| ("" + x).trim() === ""
|| isNaN(+x)) {
return false; // not a float
}
// Check for a valid integer
if (x == null
|| ("" + x).trim() === ""
|| ~~x !== +x) {
return false; // not an integer
}
function isNumberCandidate(s) {
const str = (''+ s).trim();
if (str.length === 0) return false;
return !isNaN(+str);
}
console.log(isNumberCandidate('1')); // true
console.log(isNumberCandidate('a')); // false
console.log(isNumberCandidate('000')); // true
console.log(isNumberCandidate('1a')); // false
console.log(isNumberCandidate('1e')); // false
console.log(isNumberCandidate('1e-1')); // true
console.log(isNumberCandidate('123.3')); // true
console.log(isNumberCandidate('')); // false
console.log(isNumberCandidate(' ')); // false
console.log(isNumberCandidate(1)); // true
console.log(isNumberCandidate(0)); // true
console.log(isNumberCandidate(NaN)); // false
console.log(isNumberCandidate(undefined)); // false
console.log(isNumberCandidate(null)); // false
console.log(isNumberCandidate(-1)); // true
console.log(isNumberCandidate('-1')); // true
console.log(isNumberCandidate('-1.2')); // true
console.log(isNumberCandidate(0.0000001)); // true
console.log(isNumberCandidate('0.0000001')); // true
console.log(isNumberCandidate(Infinity)); // true
console.log(isNumberCandidate(-Infinity)); // true
console.log(isNumberCandidate('Infinity')); // true
if (isNumberCandidate(s)) {
// use +s as a number
+s ...
}
Well, I'm using this one I made...
It's been working so far:
function checkNumber(value) {
return value % 1 == 0;
}
If you spot any problem with it, tell me, please.
Checking the number in JS:
Best way for check if it's a number:
isFinite(20)
//True
Read a value out of a string. CSS *:
parseInt('2.5rem')
//2
parseFloat('2.5rem')
//2.5
For an integer:
isInteger(23 / 0)
//False
If value is NaN:
isNaN(20)
//False
This way it works for me.
function isNumeric(num){
let value1 = num.toString();
let value2 = parseFloat(num).toString();
return (value1 === value2);
}
console.log(
isNumeric(123), //true
isNumeric(-123), //true
isNumeric('123'), //true
isNumeric('-123'), //true
isNumeric(12.2), //true
isNumeric(-12.2), //true
isNumeric('12.2'), //true
isNumeric('-12.2'), //true
isNumeric('a123'), //false
isNumeric('123a'), //false
isNumeric(' 123'), //false
isNumeric('123 '), //false
isNumeric('a12.2'), //false
isNumeric('12.2a'), //false
isNumeric(' 12.2'), //false
isNumeric('12.2 '), //false
)
PFB the working solution:
function(check){
check = check + "";
var isNumber = check.trim().length>0? !isNaN(check):false;
return isNumber;
}
Save yourself the headache of trying to find a "built-in" solution.
There isn't a good answer, and the hugely upvoted answer in this thread is wrong.
npm install is-number
In JavaScript, it's not always as straightforward as it should be to reliably check if a value is a number. It's common for devs to use +, -, or Number() to cast a string value to a number (for example, when values are returned from user input, regex matches, parsers, etc). But there are many non-intuitive edge cases that yield unexpected results:
console.log(+[]); //=> 0
console.log(+''); //=> 0
console.log(+' '); //=> 0
console.log(typeof NaN); //=> 'number'

RegEx how to use correctly to check str

Trying to use RegEx but doesn't work...what i've missed?How to alert message when it is "/0" in my calculator?
function div(input)
{
var input = document.getElementById("t").value;
var is_div_by_zero = /\/[\s.0]+$/.test(input);
if (is_div_by_zero)
{
alert(" / to Zero");
}
}
No need to do this with a regex, check the number value instead. E.g.
if (!Number(document.getElementById("t").value)) {
alert("Division by zero")
}
This will cast the value as a number and, when casted to a boolean, check if it's truthy or not. E.g. the value '1' will be casted to 1 which is true whereas 0 is false and 'foo' is NaN which is false.

Best way to validate a string in js

I need to make sure that my string is a number and for doing that, I was using isNaN function and everything was working but I got a problem when I typed in my input field '0e1'.
I was wondering what's the best way to check if my string is a number and without scientific notation
Try using regex. Here are some examples. The result is null if no match was found.
alert("0e1".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // null
alert("1".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // 1
alert("-1".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // -1
alert("1.0".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // 1.0
alert("-1.5".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // -1.5
alert("-1.5 4".match(/^[-+]?[0-9]*\.?[0-9]+$/)) // null
If you want your input to only be an integer or float do this instead;
var input = '0e1';
if(!isNaN(input) && (parseInt(input, 10).toString() == input || parseFloat(input).toString() == input)){
//This is a valid number
}
Actually, the method you are using is fine. You have one problem with your code:
isNaN() should be !isNaN(). Remember, isNaN() checks if the number is NotaNumber. `!isNaN() checks if it IS a number.
isNaN(0e1) should return false because 0e1 is a number! I know, it's kind of confusing.
!isNaN(0e1) will return TRUE because !isNan() checks if a value IS a number.
I hope this helps.
isNaN() function determines whether a value is an illegal number, so if it is not a number. The good thing about isNaN function is, that is suportet by all browsers (ch, ie, ff, op, sf).
function myFunction() {
var a = isNaN(123) + "<br>";
var b = isNaN(-1.23) + "<br>";
var c = isNaN(123-456) + "<br>";
var d = isNaN("Hello") + "<br>";
var e = isNaN("2014/11/19") + "<br>";
var result = a + b + c + d + e;
document.getElementById("test").innerHTML = result;
}
So result would be: If value is a number return "False" if is not a number return "True".
You can test it on JsFiddle: isNaN()

converting string into Boolean value using JavaScript always give false

I am using the code below in JavaScript to change a string on the site to a Boolean value using JavaScript. when I alert test its says its true, but when I alter name bool is see flase any ideas why this would be the case.
var test = document.getElementById("name").value;
var nameBool= (String == test);
Compare the string to the value that you expect it to have when it should represent a true value:
var nameBool = test === "true";
I would guess that test is the boolean value of a checkbox? Then you would get a textual representation of it either by
var nameBool = String(test); // type conversion
or simpler
var nameBool = "" + test; // concatenation with empty string - implicit conversion
which then becomes "false" or "true", just as in your alert() (which did a stringification as well)
For the opposite, you'd use
var test = "true" // or "false"
var nameBool = test === "true" || (test === "false" ? false : throw new SyntaxError("non-boolean string value"));

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