The basic question here is how do I know when to use and
what is the critical difference between each of them:
The Number.parseInt method (or just parseInt),
Number.parseFloat method (or just parseFloat),
Number() function (or class?),
and the + operator
for converting JavaScript values (mostly String's) to numbers.
Especially since all of them give similar values and can convert String to its Number representation:
Number.parseInt("2") // returns 2
Number.parseFloat("2") // returns 2
Number("2") // returns 2
+"2" // returns 2
/* Plus a few more methods... */
eval("2") // returns 2
JSON.parse("2") // returns 2
Number.parseInt method (or just parseInt)
Ignores leading and trailing whitespace
Parses a leading number to an integer (not a floating point number)
Ignores invalid trailing data
Lets you set the base to use when interpreting the number
Will interpret text starting with 0x as hexadecimal, if another base was not provided
Returns NaN if the value could not be successfully parsed to an integer
Number.parseFloat method (or just parseFloat)
Similar to parseInt, except that it allows for a decimal part to be interpreted
Only parses to base-10
Number() function (or class?)
Similar to parseFloat, but does not allow trailing text
Will return 0 for an empty string or a string that only contains whitespace
It's not a class; when called without new, it returns a primitive number
the + operator
Basically the same as Number(), but in operator form.
eval()
Interprets and executes the given input as a JavaScript program.
Given the string "2", it will be interpreted as a numeric literal, and return that value since it's the result of the last expression in the program
Throws an error if the input was not a valid program.
JSON.parse()
Parses the textual data as JSON-serialized data.
If the data is valid, it creates the JavaScript objects/primitives that are represented by the data, and returns them.
If the data is invalid, it throws an error.
Given the string "2", it will be interpreted as a numeric literal, and return the value that was successfully parsed out of it according to the parsing requirements of JSON.
So you decide which is appropriate to use based on their capabilities.
Number.parseInt() calls the global function parseInt() in the background, same with Number.parseFloat() see: Number.parseInt ECMA and Number.parseFloat ECMA
The calls Number("2") and "+2" is identical in the background, they both call ToNumber see: Number and Unary + Operator
When you know what types you are working with, or want a guaranteed type back, use parseFloat and parseInt, otherwise it tends to be easier to only use Number() as it will work within all your calculations, many people choose to use the unary + operator because they find it more pleasing to read/type, but that is only based on preference as it is identical to Number().
Also, when you using parseInt(), you can specify a radix, which is useful in certain applications where you want to work in different number systems, which you cannot do with Number()
If the ECMA standard references does not explain the details for you enough, I will add a summary for you.
I've always used the parseInt() function in Javascript without passing the radix parameter. As per the MDN documentation here, it's stated that not providing this parameter may result in unpredictable behaviour.
Always specify this parameter to eliminate reader confusion and to
guarantee predictable behavior.
Could somebody clarify what does this unpredictable behavior mean with some code examples?
In older versions of the language, parseInt() would cause the function to obey the normal JavaScript numeric constant syntax rules, including the recognition of a leading zero to denote octal constants, and a leading 0x to denote hex constants. Thus, if your code didn't explicitly insist on base 10, stray (possibly user-supplied) numbers with leading zeros would be interpreted as base-8 values, and leading 0x as hex.
The base-8 behavior is gone since ES5.1 (I think; might have been earlier), but the base 16 behavior is still there. (Probably a leading 0x is a little more rare as an accidental prefix than a simple leading 0.)
My experience looking at code here on Stack Overflow is that parseInt() is overused anyway. It's usually cleaner to convert strings (often, strings taken from DOM element .value properties) to numbers with the unary + operator:
var count = +document.getElementById("count").value;
That won't necessarily give you an integer, of course. However, what it will do is notice that the input string has trailing non-numeric garbage. The parseInt() function will simply stop parsing a string like "123abc" and give you 123 as the numeric value. The leading + will however give you a NaN.
If you need integers, you can always use Math.floor() or Math.round().
edit — a comment notes that ES2015 requires a leading 0o or 0O in "strict" mode for octal literals, but that doesn't apply to parseInt() which (in ES2015) only overrides the default radix for hex strings.
For some reason best known to themselves, the folk specifying the behaviour of this function set the radix to be a defaultable parameter but then decided to leave the default value up to the implementation! (Perhaps it would have been sensible to insist on a value of 10 but maybe that would have upset folk progamming in the 1970s who still consider octal literals to be useful.)
So for robust progamming, you need to supply the radix parameter yourself.
Without supplying the radix, parseInt tries to determine what the right radix is by the value you pass in, for example if the value starts 0x then it determines you must be passing in hex values. Same applies with 0 (octal).
This becomes problematic when your input is zero-padded but no radix is supplied. Where the result will (possibly) not be as expected
console.log(parseInt(015))
Example 1 of the knockout extenders page describes a way of rounding user input and making sure it is only numeric.
It works great, but looking through the source they do a peculiar thing that i don't understand, that is on line 8 they do this:
parseFloat(+newValue)
newValue is a string.
When i initially asked this question I didn't know what + did - some further poking and a link to a different MDN page from one of the initial answers I got indicate it is a unary operator equivalent to number(str) and that there are some differences between +str and parseFloat(str) (treatment of strings ending in alpha characters and interpretation of hex seem to be the headlines).
I still don't understand why the + in this case needed to be wrapped in the parseFloat although I am starting to think it might be a typo...
Citing MDN docs for parseFloat:
parseFloat parses its argument, a string, and returns a floating point number. If it encounters a character other than a sign (+ or -), numeral (0-9), a decimal point, or an exponent, it returns the value up to that point and ignores that character and all succeeding characters. Leading and trailing spaces are allowed.
Using [unary plus operator][2] you may be sure that `parseFloat` operates on `Number`, which is only useful if you want to be more strict about results but still want to use a `parseFloat`
parseFloat('0.32abcd') // -> 0.32
parseFloat(+'0.32abcd') // -> NaN
**Update:**
After a bit of digging in docs and running some tests, seems there is no reason to use parseFloat other than parsing strings that may contain numbers with non numeric trails to number, eq:
parseFloat('31.5 miles') // -> 31.5
parseFloat('12.75em') // -> 12.75
For any other cases where your string contains number + is a fastest and prefered way (citing MDN docs for unary plus operator):
unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number.
See parseFloat versus unary test case for how faster it is.
Previous link broken so here is the new test that shows how unary is faster.
My problem is really simple but I'm not sure if there's a "native" solution using JSON.parse.
I receive this string from an API :
{ "key" : -922271061845347495 }
When I'm using JSON.parse on this string, it turns into this object:
{ "key" : -922271061845347500 }
As you can see, the parsing stops when the number is too long (you can check this behavior here). It has only 15 exact digits, the last one is rounded and those after are set to 0. Is there a "native" solution to keep the exact value ? (it's an ID so I can't round it)
I know I can use regex to solve this problem but I'd prefer to use a "native" method if it exists.
Your assumption that the parsing stops after certain digits is incorrect.
It says here:
In JavaScript all numbers are floating-point numbers. JavaScript uses
the standard 8 byte IEEE floating-point numeric format, which means
the range is from:
±1.7976931348623157 x 10308 - very large, and ±5 x 10-324 - very small.
As JavaScript uses floating-point numbers the accuracy is only assured
for integers between: -9007199254740992 (-253) and 9007199254740992
(253)
You number lies outside the "accurate" range hence it is converted to the nearest representation of the JavaScript number. Any attempt to evaluate this number (using JSON.parse, eval, parseInt) will cause data loss. I therefore recommend that you pass the key as a string. If you do not control the API, file a feature request.
The number is too big to be parsed correctly.
One solution is:
Preprocessing your string from API to convert it into string before parsing.
Preform normal parsing
Optionally, you could convert it back into number for your own purpose.
Here is the RegExp to convert all numbers in your string (proceeded with :) into strings:
// convert all number fields into strings to maintain precision
// : 922271061845347495, => : "922271061845347495",
stringFromApi = stringFromApi.replace(/:\s*(-?\d+),/g, ': "$1",');
Regex explanation:
\s* any number of spaces
-? one or zero '-' symbols (negative number support)
\d+ one or more digits
(...) will be put in the $1 variable
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!