Just out of curiosity.
It doesn't seem very logical that typeof NaN is number. Just like NaN === NaN or NaN == NaN returning false, by the way. Is this one of the peculiarities of JavaScript, or would there be a reason for this?
Edit: thanks for your answers. It's not an easy thing to get ones head around though. Reading answers and the wiki I understood more, but still, a sentence like
A comparison with a NaN always returns an unordered result even when comparing with itself. The comparison predicates are either signaling or non-signaling, the signaling versions signal an invalid exception for such comparisons. The equality and inequality predicates are non-signaling so x = x returning false can be used to test if x is a quiet NaN.
just keeps my head spinning. If someone can translate this in human (as opposed to, say, mathematician) readable language, I would be grateful.
Well, it may seem a little strange that something called "not a number" is considered a number, but NaN is still a numeric type, despite that fact :-)
NaN just means the specific value cannot be represented within the limitations of the numeric type (although that could be said for all numbers that have to be rounded to fit, but NaN is a special case).
A specific NaN is not considered equal to another NaN because they may be different values. However, NaN is still a number type, just like 2718 or 31415.
As to your updated question to explain in layman's terms:
A comparison with a NaN always returns an unordered result even when comparing with itself. The comparison predicates are either signalling or non-signalling, the signalling versions signal an invalid exception for such comparisons. The equality and inequality predicates are non-signalling so x = x returning false can be used to test if x is a quiet NaN.
All this means is (broken down into parts):
A comparison with a NaN always returns an unordered result even when comparing with itself.
Basically, a NaN is not equal to any other number, including another NaN, and even including itself.
The comparison predicates are either signalling or non-signalling, the signalling versions signal an invalid exception for such comparisons.
Attempting to do comparison (less than, greater than, and so on) operations between a NaN and another number can either result in an exception being thrown (signalling) or just getting false as the result (non-signalling or quiet).
The equality and inequality predicates are non-signalling so x = x returning false can be used to test if x is a quiet NaN.
Tests for equality (equal to, not equal to) are never signalling so using them will not cause an exception. If you have a regular number x, then x == x will always be true. If x is a NaN, then x == x will always be false. It's giving you a way to detect NaN easily (quietly).
It means Not a Number. It is not a peculiarity of javascript but common computer science principle.
From http://en.wikipedia.org/wiki/NaN:
There are three kinds of operation
which return NaN:
Operations with a NaN as at least one operand
Indeterminate forms
The divisions 0/0, ∞/∞, ∞/−∞, −∞/∞, and −∞/−∞
The multiplications 0×∞ and 0×−∞
The power 1^∞
The additions ∞ + (−∞), (−∞) + ∞ and equivalent subtractions.
Real operations with complex results:
The square root of a negative number
The logarithm of a negative number
The tangent of an odd multiple of 90 degrees (or π/2 radians)
The inverse sine or cosine of a number which is less than −1 or
greater than +1.
All these values may not be the same. A simple test for a NaN is to test value == value is false.
The ECMAScript (JavaScript) standard specifies that Numbers are IEEE 754 floats, which include NaN as a possible value.
ECMA 262 5e Section 4.3.19: Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value.
ECMA 262 5e Section 4.3.23: NaN
Number value that is a IEEE 754 "Not-a-Number" value.
IEEE 754 on Wikipedia
The IEEE Standard for Floating-Point Arithmetic is a technical standard established by the Institute of Electrical and Electronics Engineers and the most widely used standard for floating-point computation [...]
The standard defines
arithmetic formats: sets of binary and decimal floating-point data, which consist of finite numbers (including signed zeros and subnormal numbers), infinities, and special "not a number" values (NaNs)
[...]
typeof NaN returns 'number' because:
ECMAScript spec says the Number type includes NaN:
4.3.20 Number type
set of all possible Number values including the special “Not-a-Number”
(NaN) values, positive infinity, and negative infinity
So typeof returns accordingly:
11.4.3 The typeof Operator
The production UnaryExpression : typeof UnaryExpression is
evaluated as follows:
Let val be the result of evaluating UnaryExpression.
If Type(val) is Reference, then
If IsUnresolvableReference(val) is true, return "undefined".
Let val be GetValue(val).
Return a String determined by Type(val) according to Table 20.
Table 20 — typeof Operator Results
==================================================================
| Type of val | Result |
==================================================================
| Undefined | "undefined" |
|----------------------------------------------------------------|
| Null | "object" |
|----------------------------------------------------------------|
| Boolean | "boolean" |
|----------------------------------------------------------------|
| Number | "number" |
|----------------------------------------------------------------|
| String | "string" |
|----------------------------------------------------------------|
| Object (native and does | "object" |
| not implement [[Call]]) | |
|----------------------------------------------------------------|
| Object (native or host and | "function" |
| does implement [[Call]]) | |
|----------------------------------------------------------------|
| Object (host and does not | Implementation-defined except may |
| implement [[Call]]) | not be "undefined", "boolean", |
| | "number", or "string". |
------------------------------------------------------------------
This behavior is in accordance with IEEE Standard for Floating-Point Arithmetic (IEEE 754):
4.3.19 Number value
primitive value corresponding to a double-precision 64-bit binary
format IEEE 754 value
4.3.23 NaN
number value that is a IEEE 754 “Not-a-Number” value
8.5 The Number Type
The Number type has exactly 18437736874454810627 (that is, 253−264+3)
values, representing the double-precision 64-bit format IEEE 754
values as specified in the IEEE Standard for Binary Floating-Point
Arithmetic, except that the 9007199254740990 (that is, 253−2) distinct
“Not-a-Number” values of the IEEE Standard are represented in
ECMAScript as a single special NaN value. (Note that the NaN value
is produced by the program expression NaN.)
NaN != NaN because they are not necessary the SAME non-number. Thus it makes a lot of sense...
Also why floats have both +0.00 and -0.00 that are not the same. Rounding may do that they are actually not zero.
As for typeof, that depends on the language. And most languages will say that NaN is a float, double or number depending on how they classify it... I know of no languages that will say this is an unknown type or null.
NaN is a valid floating point value (http://en.wikipedia.org/wiki/NaN)
and NaN === NaN is false because they're not necessarily the same non-number
NaN stands for Not a Number. It is a value of numeric data types (usually floating point types, but not always) that represents the result of an invalid operation such as dividing by zero.
Although its names says that it's not a number, the data type used to hold it is a numeric type. So in JavaScript, asking for the datatype of NaN will return number (as alert(typeof(NaN)) clearly demonstrates).
A better name for NaN, describing its meaning more precisely and less confusingly, would be a numerical exception. It is really another kind of exception object disguised as having primitive type (by the language design), where at the same it is not treated as primitive in its false self-comparison. Whence the confusion. And as long as the language "will not make its mind" to choose between proper exception object and primitive numeral, the confusion will stay.
The infamous non-equality of NaN to itself, both == and === is a manifestation of the confusing design forcing this exception object into being a primitive type. This breaks the fundamental principle that a primitive is uniquely determined by its value. If NaN is preferred to be seen as exception (of which there can be different kinds), then it should not be "sold" as primitive. And if it is wanted to be primitive, that principle must hold. As long as it is broken, as we have in JavaScript, and we can't really decide between the two, the confusion leading to unnecessary cognitive load for everyone involved will remain. Which, however, is really easy to fix by simply making the choice between the two:
either make NaN a special exception object containing the useful information about how the exception arose, as opposed to throwing that information away as what is currently implemented, leading to harder-to-debug code;
or make NaN an entity of the primitive type number (that could be less confusingly called "numeric"), in which case it should be equal to itself and cannot contain any other information; the latter is clearly an inferior choice.
The only conceivable advantage of forcing NaN into number type is being able to throw it back into any numerical expression. Which, however, makes it brittle choice, because the result of any numerical expression containing NaN will either be NaN, or leading to unpredictable results such as NaN < 0 evaluating to false, i.e. returning boolean instead of keeping the exception.
And even if "things are the way they are", nothing prevents us from making that clear distinction for ourselves, to help make our code more predictable and easierly debuggable. In practice, that means identifying those exceptions and dealing with them as exceptions. Which, unfortunately, means more code but hopefully will be mitigated by tools such as TypeScript of Flowtype.
And then we have the messy quiet vs noisy aka signalling NaN distinction. Which really is about how exceptions are handled, not the exceptions themselves, and nothing different from other exceptions.
Similarly, Infinity and +Infinity are elements of numeric type arising in the extension of the real line but they are not real numbers. Mathematically, they can be represented by sequences of real numbers converging to either + or -Infinity.
Javascript uses NaN to represent anything it encounters that can't be represented any other way by its specifications. It does not mean it is not a number. It's just the easiest way to describe the encounter. NaN means that it or an object that refers to it could not be represented in any other way by javascript. For all practical purposes, it is 'unknown'. Being 'unknown' it cannot tell you what it is nor even if it is itself. It is not even the object it is assigned to. It can only tell you what it is not, and not-ness or nothingness can only be described mathematically in a programming language. Since mathematics is about numbers, javascript represents nothingness as NaN. That doesn't mean it's not a number. It means we can't read it any other way that makes sense. That's why it can't even equal itself. Because it doesn't.
This is simply because NaN is a property of the Number object in JS, It has nothing to do with it being a number.
The best way to think of NAN is that its not a known number. Thats why NAN != NAN because each NAN value represents some unique unknown number. NANs are necessary because floating point numbers have a limited range of values. In some cases rounding occurs where the lower bits are lost which leads to what appears to be nonsense like 1.0/11*11 != 1.0. Really large values which are greater are NANs with infinity being a perfect example.
Given we only have ten fingers any attempt to show values greater than 10 are impossible, which means such values must be NANs because we have lost the true value of this greater than 10 value. The same is true of floating point values, where the value exceeds the limits of what can be held in a float.
NaN is still a numeric type, but it represents value that could not represent a valid number.
Because NaN is a numeric data type.
NaN is a number from a type point of view, but is not a normal number like 1, 2 or 329131. The name "Not A Number" refers to the fact that the value represented is special and is about the IEEE format spec domain, not javascript language domain.
If using jQuery, I prefer isNumeric over checking the type:
console.log($.isNumeric(NaN)); // returns false
console.log($.type(NaN)); // returns number
http://api.jquery.com/jQuery.isNumeric/
Javascript has only one numeric data type, which is the standard 64-bit double-precision float. Everything is a double. NaN is a special value of double, but it's a double nonetheless.
All that parseInt does is to "cast" your string into a numeric data type, so the result is always "number"; only if the original string wasn't parseable, its value will be NaN.
We could argue that NaN is a special case object. In this case, NaN's object represents a number that makes no mathematical sense. There are some other special case objects in math like INFINITE and so on.
You can still do some calculations with it, but that will yield strange behaviours.
More info here: http://www.concentric.net/~ttwang/tech/javafloat.htm (java based, not javascript)
You've got to love Javascript. It has some interesting little quirks.
http://wtfjs.com/page/13
Most of those quirks can be explained if you stop to work them out logically, or if you know a bit about number theory, but nevertheless they can still catch you out if you don't know about them.
By the way, I recommend reading the rest of http://wtfjs.com/ -- there's a lot more interesting quirks than this one to be found!
The value NaN is really the Number.NaN hence when you ask if it is a number it will say yes. You did the correct thing by using the isNaN() call.
For information, NaN can also be returned by operations on Numbers that are not defined like divisions by zero or square root of a negative number.
It is special value of Number type as POSITIVE_INFINITY
Why? By design
An example
Imagine We are converting a string to a number:
Number("string"); // returns NaN
We changed the data type to number but its value is not a number!
Related
I want to define a BigInt number in JavaScript. But when I assign it, the wrong number is stored. In fact 1 is added to the number when storing.
let num = BigInt(0b0000111111111111111111111111111111111111111111111111111111111111)
console.log(num) // Output: 1152921504606846976n
console.log(num.toString(2)) // Output: 1000000000000000000000000000000000000000000000000000000000000
So the number stored is 1152921504606846976, but it should be 11529215046068469765. Why is that?
Converting a Number to a BigInt can't create bits that weren't there before.
0b1 (just like 1) is a Number literal, so it creates a Number.
0b1n (just like 1n) is a BigInt literal, so it creates a BigInt.
By writing BigInt(0b1), you're first creating a Number and then converting that to a BigInt. As long as the value is 1, that works just fine; once the value exceeds what you can losslessly store in a Number [1], you'll see that the value of the final BigInt won't match the literal you wrote down. Whether you use binary (0b...), decimal, or hex (0x...) literals doesn't change any of that.
(And just to be extra clear: there's no reason to write BigInt(123n), just like you wouldn't write Number(123). 123n already is a BigInt, so there's nothing to convert.)
A simple non-BigInt way to illustrate what's happening is to enter 12345678901234567890 into your favorite browser's DevTools console: you can specify Number literals of any length you want, but they'll be parsed into an IEEE754 64-bit "double", which has limited precision. Any extra digits in the literal simply can't be stored, though of course each digit's presence affects the magnitude of the number.
[1] Side note: this condition is more subtle than just saying that Number.MAX_SAFE_INTEGER is the threshold, though that constant is related to the situation: any integral number below MAX_SAFE_INTEGER can be stored losslessly, but there are plenty of numbers above MAX_SAFE_INTEGER that can also be represented exactly. Random example: 1e20.
The problem: JavaScript returns 20160107190308484 for parseInt("20160107190308485")
Not the question: I do not need a workaround; I got one.
Not the question II: I do not need an explanation why this can happen, and how numbers are stored and so on.
How are numbers as input for parseInt defined? Why do I get a wrong result, not a NaN?
I only find in documentation how parseInt handles literals and so on, that only the first numbers are interpreted and ...
As W3C or MDN
Note: Only the first number in the string is returned!
Note: Leading and trailing spaces are allowed.
Note: If the first character cannot be converted to a number, parseInt() returns NaN.
I can't find anything that says "123456789202232334234212312311" is not a "number". And nothing like "a number must be in the range of -BIGINT .. +BIGINT or so. And I could not find a hint for errors thrown.
I just get 20160107190308484 for "20160107190308485" (two browsers tested) and other pairs like:
20160107155044520, 20160107155044522
20160107002720970, 20160107002720967
20160107000953860, 20160107000953859
For me this feels like a hole in ECMA.
Is there a number defined that is allowed as input for parseInt?
For those who are interested in "how came?":
I stumbled over that problem with a small logic, that converts classes and ids into an object holding the information like
class="book240 lang-en" id="book240page2212"
converting to an object like:
{
book: 240
page: 2212
lang: "en"
}
Notice, that INTs are numbers, not strings.
Therefore I have a loop that makes that generic:
for n in some_regexp_check_names
# m hold a Regexp result arg[1] is the interest:
nr = parseInt(m[1]) # Make it a number
#--------- Here the funny fault
if nr > 0 # Check if number & > 0 all my ids are > 0
out_obj[n]=nr # Take the number
else
out_obj[n]=m[1] # Take the string as is
All this is nice and is working with "real strings" and "real (int) ids", and then I had a situation where dynamically semi uuids out of datetime where created: "temp-2016-01-07-19-03-08.485" - and still all fine, but I had the idea to remove all constant chars from this (i.e. temp-.) and BANG, because the string "20160107190308485" gives 20160107190308484 as parseInt.
And it took me only ~3 hours to find that error.
How are numbers as input for parseInt defined?
I think it's pretty clearly defined in the spec (§18.2.5):
Let mathInt be the mathematical integer value that is represented by Z
in radix-R notation, using the letters A-Z and a-z for digits with
values 10 through 35. (However, if R is 10 and Z contains more than 20
significant digits, every significant digit after the 20th may be
replaced by a 0 digit, at the option of the implementation; and if R
is not 2, 4, 8, 10, 16, or 32, then mathInt may be an
implementation-dependent approximation to the mathematical integer
value that is represented by Z in radix-R notation.)
Let number be the Number value for mathInt.
For that last step, the section for the Number type (§6.1.6) specifies:
In this specification, the phrase “the Number value for x” where x
represents an exact nonzero real mathematical quantity (which might
even be an irrational number such as π) means a Number value chosen in
the following manner. Consider the set of all finite values of the
Number type, with −0 removed and with two additional values added to
it that are not representable in the Number type, namely 21024 (which
is +1 × 253 × 2971) and −21024 (which is −1 × 253 × 2971). Choose the
member of this set that is closest in value to x. If two values of the
set are equally close, then the one with an even significand is
chosen; for this purpose, the two extra values 21024 and −21024 are
considered to have even significands. Finally, if 21024 was chosen,
replace it with +∞; if −21024 was chosen, replace it with −∞; if +0
was chosen, replace it with −0 if and only if x is less than zero; any
other chosen value is used unchanged. The result is the Number value
for x. (This procedure corresponds exactly to the behaviour of the
IEEE 754-2008 “round to nearest, ties to even” mode.)
Why do I get a wrong result not a NaN?
You get a rounded result (with the maximum precision available in that range) - and there's nothing wrong with that, your input is a valid number, not NaN.
Although Bergis answer is correct, I want to tell what my real mistake was:
Don't think of parseInt of a function, that parses into an INTeger like known from C as 16, 32 or 64 bit value. It parse the number as long as there are valid digits, so if the number is to big (does not fit exactly into eg 64 bit), you dont get an error, you just get a rounded value like working with float. If you want to be on the safe side: check the result against Number.MAX_SAFE_INTEGER.
The maximum integer value in JavaScript is 2^53 == 9 007 199 254 740 992. This is because Numbers are stored as floating point is a 52 bit mantissa.
Read more: http://web.archive.org/web/20161117214618/http://bjola.ca/coding/largest-integer-in-javascript/
I realized that in javascript all 101/100, "101"/100, 101/"100" and "101"/"100" result in 1.01 (checked on Chrome, FF and IE11). But I cannot find a piece of documentation regarding this behaviour.
Therefore my question is if it is (cross-browser) safe to use this feature, and if it is a good practice to do so (or rather to use parseInt before division if the variable can be a string)?
When you use / on strings, the strings are implicitly converted to numbers and then division operation is performed.
This may work in all browsers, but it's always good practice to convert to number explicitly using parseInt or parseFloat or other method.
parseInt("101", 10) / 100
Or
parseFloat("101") / 100
ECMAScript Specifications for Division Operator
Therefore my question is if it is (cross-browser) safe to use this feature...
It depends on your definition of "safe." With the division operator, yes, it's specified behavior: Each operand is converted (implicitly coerced) to a number, and then numeric division is done.
But beware of generalizing this too far. You'll be okay with /, *, and - but it will bite you on +, because if either operand to + is a string, + does string concatenation, not addition.
Another way that it may or may not be "safe" depending on your point of view is the implicit coercion: It uses the browser's JavaScript engine's rules for converting strings to numbers. Some older browsers went beyond the specification (which they were allowed to in the past) and treated numbers starting with a 0 as octal (base 8) rather than decimal. Naturally, end users who type in, say, "0123" as a number probably mean the number 123, not the number 83 (123 in octal = 83 decimal). JavaScript engines are no longer allowed to do that, but some older ones do.
In general, it's probably best to explicitly coerce or convert those operands. Your choices for doing so:
The unary + operator: value = +value will coerce the string to a number using the JavaScript engine's standard rules for that. Any non-digits in the string (other than the e for scientific notation) make the result NaN. Also, +"" is 0, which may not be intuitive.
The Number function: value = Number(value). Does the same thing as +.
The parseInt function, usually with a radix (number base): value = parseInt(value, 10). The downside here is that parseInt converts any number it finds at the beginning of the string but ignores non-digits later in the string, so parseInt("100asdf", 10) is 100, not NaN. As the name implies, parseInt parses only a whole number.
The parseFloat function: value = parseFloat(value). Allows fractional values, and always works in decimal (never octal or hex). Does the same thing parseInt does with garbage at the end of the string, parseFloat("123.34alksdjf") is 123.34.
So, pick your tool to suit your use case. :-)
Type coercion is at play here. Quoting #Barmar's answer from What exactly is Type Coercion in Javascript?
Type coercion means that when the operands of an operator are of different types, one of them will be converted to an "equivalent" value of the other operand's type.
The reason for your observation is valid for other operations too -
1 + "2" will give you "12"
1 - "2" will give you -1
(because "-" operation on strings is not defined like division")
In the case "101/100" the operation "/" will decide the coercion, since there is no operation defined on strings with that operator "/", but is there for "numbers".
Using it is safe (at least in modern browsers) as long as you are clear how type coercion will play out in your operation.
This question already has answers here:
Why does typeof NaN return 'number'?
(21 answers)
Closed 7 years ago.
NaN represents Not-A-Number.
It appears that angular.isNumber thinks it is a number. (angularjs 1.4.2)
Why does angular.isNumber return true for NaN input?
thanks
Quoting IgorMinar, Angular Developer in this exact question:
$ node
> typeof NaN
'number'
It kind of makes sense if you squint with both eyes and plug your
ears.
If you deliberately use NaN in your app, you should use isNaN instead
of angular.isNumber.
I'm inclined to say that the current behavior, even though a bit
surprising, is consistent with how NaN is being treated in javascript.
If you have some good arguments for changing the behavior please share
them with us.
So the question really goes for the javascript standard itself not for Angular
And to answer this question we must go to ECMAScript 5 specification of number type, of course it says:
4.3.20 Number type
set of all possible Number values including the special “Not-a-Number”
(NaN) values, positive infinity, and negative infinity
4.3.23 NaN
number value that is a IEEE 754 “Not-a-Number” value
So yes, according to the latest ECMAScript Specification i'm a number
Here's the best way that I can think of to explain this.
Although the value of NaN represents something that is not a number, the value NaN itself is still a number type (in the type system sense).
It's also a defined value for a floating point number in IEEE 754, which is what JavaScript uses for numbers. It is sensible that values infinity and NaN would be number types.
The ECMA spec defines NaN as a IEEE 754 Not-a-Number number value. One reason for the NaN global being a number are comparison purposes. It is also needed to represent undefined numerical results, like the value of Math.sqrt(-1). So it’s not particularly AngularJS specific. Consider the following:
typeof NaN === "number" // true
typeof NaN === typeof NaN // true
typeof NaN === typeof 123 // true
NaN === NaN // false
isNaN(NaN) // true
isNaN(123) // false
isNaN('123') // false
isNaN('|23') // true
So isNumber returns true for NaN because it is a Number. To check for numerics, use isNaN().
Most likely angular just uses the type of what you pass in. if the type is number then it returns true.
If you want to know if something is a number (excluding NaN) you can do the following.
function isNumber(val){
return angular.isNumber(val) && (val == val);
}
This works by first determing if val is a number. If it is check to see if it's NaN.
NaN is not equal to itself (or any other number for that matter).
it's not related to angular, it's JavaScript
try this
typeof NaN
it will return number
There are really two meanings of "number" here:
the abbreviation "NaN" means that there is no meaningful answer to a particular mathematical operation; you could say "no such number"
however, every value in a language like JS has a type, and the type of data which goes into and out of mathematical operations is known in JS as "number"; thus when JS wants a special value to say that a mathematical operation has no answer, that special value is a special number
Note that this apparent contradiction is less obvious in other languages, because JS is unusual in having only one numeric type, rather than (at least) integer and real/float types. Having NaN as a floating point value is standard across pretty much all modern languages, but because of the word "number", it perhaps seems more surprising in JS.
The Angular function is one of a set of utilities for testing the type of a value. The documentation for it mentions that it returns true for infinities and NaN, and points to the standard isFinite function for when that's not desirable.
In JavaScript, variables are loosely typed, so the number 5 and the string "5" may both be treated as a number by several operators. However, is there a generic way to find out JavaScripts conversion abilites in at tunrime, or is it just the overloading of operators for several types that make the loose typing possible?
For example, given a variable a and a string containing a type name type_canditate, is there any way to ask JavaScript, if a may convert to type_candidate in a feasable manner, in contrast to the hard typing operators like instanceof? For example, "5" instanceof Number evaluates false, while Math.sin("5") is perfectly feasable. For numbers, one can obviuosly check if parseFloat(some_number) evaluates to NaN, but this is a special case for numbers.
So, is there any generic way of naming types, and check if some variable may convert to a given type in a useful manner?
There are three primitive data types in JavaScript: string, number and boolean.
Anything can be converted to a string or boolean:
All objects convert to true (except null, which becomes false - I only mention it here because typeof null gives object)
All objects have a built-in toString method which is called when converting to a string.
Converting a number to a string is done by giving the string representation of the number (ie. 5 becomes "5")
Numbers convert to boolean true, unless it's 0 which becomes false.
Converting to a number is a little trickier, but technically possible. If it can find a valid number, then it becomes that number. Otherwise, it becomes NaN.
So basically... any type can become any other type through casting in this way. The only time you have anything resembling an "error condition" is NaN.