I want to check if a number is negative. I’m searching for the easiest way, so a predefined JavaScript function would be the best, but I didn’t find anything yet. Here is what I have so far, but I don’t think that this is a good way:
function negative(number) {
if (number.match(/^-\d+$/)) {
return true;
} else {
return false;
}
}
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.
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).
<)?: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.
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.
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");
}
How about something as simple as:
function negative(number){
return number < 0;
}
The * 1 part is to convert strings to numbers.
If you really want to dive into it and even need to distinguish between -0 and 0, here's a way to do it.
function negative(number) {
return !Object.is(Math.abs(number), +number);
}
console.log(negative(-1)); // true
console.log(negative(1)); // false
console.log(negative(0)); // false
console.log(negative(-0)); // true
In ES6 you can use Math.sign function to determine if,
1. its +ve no
2. its -ve no
3. its zero (0)
4. its NaN
console.log(Math.sign(1)) // prints 1
console.log(Math.sign(-1)) // prints -1
console.log(Math.sign(0)) // prints 0
console.log(Math.sign("abcd")) // prints NaN
An nice way that also checks for positive and negative also...
function ispositive(n){
return 1/(n*0)===1/0
}
console.log( ispositive(10) ) //true
console.log( ispositive(-10) ) //false
console.log( ispositive(0) ) //true
console.log( ispositive(-0) ) //false
essentially compares Infinity with -Infinity because 0===-0// true
