Javascript negative number
Instead of writing a function to do this check, you should just be able to use this expression:
(number < 0)
Javascript will evaluate this expression by first trying to convert the left hand side to a number value before checking if it's less than zero, which seems to be what you wanted.
Specifications and details
The behavior for x < y
is specified in §11.8.1 The Less-than Operator (<
), which uses §11.8.5 The Abstract Relational Comparison Algorithm.
The situation is a lot different if both x
and y
are strings, but since the right hand side is already a number in (number < 0)
, the comparison will attempt to convert the left hand side to a number to be compared numerically. If the left hand side can not be converted to a number, the result is false
.
Do note that this may give different results when compared to your regex-based approach, but depending on what is it that you're trying to do, it may end up doing the right thing anyway.
"-0" < 0
isfalse
, which is consistent with the fact that-0 < 0
is alsofalse
(see: signed zero)."-Infinity" < 0
istrue
(infinity is acknowledged)"-1e0" < 0
istrue
(scientific notation literals are accepted)"-0x1" < 0
istrue
(hexadecimal literals are accepted)" -1 " < 0
istrue
(some forms of whitespaces are allowed)
For each of the above example, the regex method would evaluate to the contrary (true
instead of false
and vice versa).
References
- ECMAScript 5 (PDF)
- ECMAScript 3, §11.8.1 The Less-than Operator (
<
) - ECMAScript 3, §11.8.5 The Abstract Relational Comparison Algorithm
See also
- regular-expressions.info/Matching Floating Point Numbers with a Regular Expression
Appendix 1: Conditional operator ?:
It should also be said that statements of this form:
if (someCondition) {
return valueForTrue;
} else {
return valueForFalse;
}
can be refactored to use the ternary/conditional ?:
operator (§11.12) to simply:
return (someCondition) ? valueForTrue : valueForFalse;
Idiomatic usage of ?:
can make the code more concise and readable.
Related questions
- javascript if alternative
- To Ternary or Not To Ternary?
Appendix 2: Type conversion functions
Javascript has functions that you can call to perform various type conversions.
Something like the following:
if (someVariable) {
return true;
} else {
return false;
}
Can be refactored using the ?:
operator to:
return (someVariable ? true : false);
But you can also further simplify this to:
return Boolean(someVariable);
This calls Boolean
as a function (§15.16.1) to perform the desired type conversion. You can similarly call Number
as a function (§15.17.1) to perform a conversion to number.
Related questions
- How can I convert a string to boolean in JavaScript?
- what is the purpose of javascript new Boolean() ?
function negative(n) {
return n < 0;
}
Your regex should work fine for string numbers, but this is probably faster. (edited from comment in similar answer above, conversion with +n
is not needed.)
This is an old question but it has a lot of views so I think that is important to update it.
ECMAScript 6 brought the function Math.sign()
, which returns the sign of a number (1 if it's positive, -1 if it's negative) or NaN if it is not a number. Reference
You could use it as:
var number = 1;
if(Math.sign(number) === 1){
alert("I'm positive");
}else if(Math.sign(number) === -1){
alert("I'm negative");
}else{
alert("I'm not a number");
}