Check whether variable is number or string in JavaScript
If you're dealing with literal notation, and not constructors, you can use typeof:.
typeof "Hello World"; // string
typeof 123; // number
If you're creating numbers and strings via a constructor, such as var foo = new String("foo")
, you should keep in mind that typeof
may return object
for foo
.
Perhaps a more foolproof method of checking the type would be to utilize the method found in underscore.js (annotated source can be found here),
var toString = Object.prototype.toString;
_.isString = function (obj) {
return toString.call(obj) == '[object String]';
}
This returns a boolean true
for the following:
_.isString("Jonathan"); // true
_.isString(new String("Jonathan")); // true
How to check if a variable is an integer or a string?
In my opinion you have two options:
Just try to convert it to an
int
, but catch the exception:try:
value = int(value)
except ValueError:
pass # it was a string, not an int.This is the Ask Forgiveness approach.
Explicitly test if there are only digits in the string:
value.isdigit()
str.isdigit()
returnsTrue
only if all characters in the string are digits (0
-9
).The
unicode
/ Python 3str
type equivalent isunicode.isdecimal()
/str.isdecimal()
; only Unicode decimals can be converted to integers, as not all digits have an actual integer value (U+00B2 SUPERSCRIPT 2 is a digit, but not a decimal, for example).This is often called the Ask Permission approach, or Look Before You Leap.
The latter will not detect all valid int()
values, as whitespace and +
and -
are also allowed in int()
values. The first form will happily accept ' +10 '
as a number, the latter won't.
If your expect that the user normally will input an integer, use the first form. It is easier (and faster) to ask for forgiveness rather than for permission in that case.
Check if a variable is a string in JavaScript
You can use typeof
operator:
var booleanValue = true;
var numericalValue = 354;
var stringValue = "This is a String";
var stringObject = new String( "This is a String Object" );
alert(typeof booleanValue) // displays "boolean"
alert(typeof numericalValue) // displays "number"
alert(typeof stringValue) // displays "string"
alert(typeof stringObject) // displays "object"
Example from this webpage. (Example was slightly modified though).
This won't work as expected in the case of strings created with new String()
, but this is seldom used and recommended against[1][2]. See the other answers for how to handle these, if you so desire.
- The Google JavaScript Style Guide says to never use primitive object wrappers.
- Douglas Crockford recommended that primitive object wrappers be deprecated.
How to check if a variable is a number or a string?
There are several ways:
>> 1.class #=> Fixnum
>> "foo".class #=> String
>> 1.is_a? Numeric #=> true
>> "foo".is_a? String #=> true
How to check if a String is numeric in Java
With Apache Commons Lang 3.5 and above: NumberUtils.isCreatable
or StringUtils.isNumeric
.
With Apache Commons Lang 3.4 and below: NumberUtils.isNumber
or StringUtils.isNumeric
.
You can also use StringUtils.isNumericSpace
which returns true
for empty strings and ignores internal spaces in the string. Another way is to use NumberUtils.isParsable
which basically checks the number is parsable according to Java. (The linked javadocs contain detailed examples for each method.)
Identify if a string is a number
int n;
bool isNumeric = int.TryParse("123", out n);
Update As of C# 7:
var isNumeric = int.TryParse("123", out int n);
or if you don't need the number you can discard the out parameter
var isNumeric = int.TryParse("123", out _);
The var s can be replaced by their respective types!
How can I check if a string is a valid number?
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
Detect whether a Python string is a number or a letter
Check if string is nonnegative digit (integer) and alphabet
You may use str.isdigit()
and str.isalpha()
to check whether a given string is a nonnegative integer (0 or greater) and alphabetical character, respectively.
Sample Results:
# For alphabet
>>> 'A'.isdigit()
False
>>> 'A'.isalpha()
True
# For digit
>>> '1'.isdigit()
True
>>> '1'.isalpha()
False
Check for strings as positive/negative - integer/float
str.isdigit()
returns False
if the string is a negative number or a float number. For example:
# returns `False` for float
>>> '123.3'.isdigit()
False
# returns `False` for negative number
>>> '-123'.isdigit()
False
If you want to also check for the negative integers and float
, then you may write a custom function to check for it as:
def is_number(n):
try:
float(n) # Type-casting the string to `float`.
# If string is not a valid `float`,
# it'll raise `ValueError` exception
except ValueError:
return False
return True
Sample Run:
>>> is_number('123') # positive integer number
True
>>> is_number('123.4') # positive float number
True
>>> is_number('-123') # negative integer number
True
>>> is_number('-123.4') # negative `float` number
True
>>> is_number('abc') # `False` for "some random" string
False
Discard "NaN" (not a number) strings while checking for number
The above functions will return True
for the "NAN" (Not a number) string because for Python it is valid float representing it is not a number. For example:
>>> is_number('NaN')
True
In order to check whether the number is "NaN", you may use math.isnan()
as:
>>> import math
>>> nan_num = float('nan')
>>> math.isnan(nan_num)
True
Or if you don't want to import additional library to check this, then you may simply check it via comparing it with itself using ==
. Python returns False
when nan
float is compared with itself. For example:
# `nan_num` variable is taken from above example
>>> nan_num == nan_num
False
Hence, above function is_number
can be updated to return False
for "NaN"
as:
def is_number(n):
is_number = True
try:
num = float(n)
# check for "nan" floats
is_number = num == num # or use `math.isnan(num)`
except ValueError:
is_number = False
return is_number
Sample Run:
>>> is_number('Nan') # not a number "Nan" string
False
>>> is_number('nan') # not a number string "nan" with all lower cased
False
>>> is_number('123') # positive integer
True
>>> is_number('-123') # negative integer
True
>>> is_number('-1.12') # negative `float`
True
>>> is_number('abc') # "some random" string
False
Allow Complex Number like "1+2j" to be treated as valid number
The above function will still return you False
for the complex numbers. If you want your is_number
function to treat complex numbers as valid number, then you need to type cast your passed string to complex()
instead of float()
. Then your is_number
function will look like:
def is_number(n):
is_number = True
try:
# v type-casting the number here as `complex`, instead of `float`
num = complex(n)
is_number = num == num
except ValueError:
is_number = False
return is_number
Sample Run:
>>> is_number('1+2j') # Valid
True # : complex number
>>> is_number('1+ 2j') # Invalid
False # : string with space in complex number represetantion
# is treated as invalid complex number
>>> is_number('123') # Valid
True # : positive integer
>>> is_number('-123') # Valid
True # : negative integer
>>> is_number('abc') # Invalid
False # : some random string, not a valid number
>>> is_number('nan') # Invalid
False # : not a number "nan" string
PS: Each operation for each check depending on the type of number comes with additional overhead. Choose the version of is_number
function which fits your requirement.
What's the easiest way to check if a javascript variable is a number and greater than zero?
You are doing a lot of checking for things that it is not, when you only care if it is a number greater than zero.
Just:
return typeof a === "number" && a > 0
Related Topics
How to Chop a String into Chunks of a Given Length in Ruby
Access Instance Variable from Outside the Class
One Liner in Ruby for Displaying a Prompt, Getting Input, and Assigning to a Variable
Render an Erb Template with Values from a Hash
Convert an Array of Integers into an Array of Strings in Ruby
Why Bundle Install Is Installing Gems in Vendor/Bundle
Ruby: How to Make Irb Print Structure for Arrays and Hashes
Undefined Instance Method "Respond_To" in Rails 5 API Controller
How to Specify a Required Switch (Not Argument) with Ruby Optionparser
Rbenv: Surviving Without Gemsets
How to Convert a String to a Class Method
Transforming Datetime into Month, Day and Year
How to Do Basic Authentication with Restclient
Are the Date, Time, and Datetime Classes Necessary
Aws Elastic Beanstalk - How to Upgrade Existing Environment from Ruby 2.1 to Ruby 2.2
Which Algorithm Does Ruby's Sort Method Use