How to check if a number is negative?

Solution 1:

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 is false, which is consistent with the fact that -0 < 0 is also false (see: signed zero).
• "-Infinity" < 0 is true (infinity is acknowledged)
• "-1e0" < 0 is true (scientific notation literals are accepted)
• "-0x1" < 0 is true (hexadecimal literals are accepted)
• " -1 " < 0 is true (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() ?

Solution 2:

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.)

Solution 3:

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");
}