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Cover image for Math.sign: How to Check if a Number is Positive or Negative in JavaScript
Samantha Ming
Samantha Ming

Posted on • Updated on • Originally published at samanthaming.com

Math.sign: How to Check if a Number is Positive or Negative in JavaScript

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Determining the sign of a number is super easy now with ES6's Math.sign 👏 It will indicate whether the number is positive, negative or zero.

const positive = 5;
const negative = -5;
const zero = 0;

Math.sign(positive); // 1
Math.sign(negative); // -1
Math.sign(zero); // 0

Math.sign Return Value

Math.sign() has 5 possible return values:

1 // positive number
-1 // negative number
0 // positive zero
-0 // negative zero
NaN // not a number
Math.sign(8); // 1
Math.sign(-8); // -1

Math.sign(0); // 0
Math.sign(-0); // -0

Math.sign(NaN); // NaN
Math.sign('hello'); // NaN
Math.sign(); //NaN

Note, the argument passed to this function will be converted to number type implicitly.

Math.sign Gotcha

A common gotcha is thinking that Math.sign return the converted argument value. Math.sign returns only the sign of a number. It doesn't return the value.

Math.sign(-8);

// ✅ return -1
// ❌ It doesn't return -8

Math.sign vs Comparative Operator

Got a real good question from the community:

Why use Math.sign when I can use the comparative operator?

if (number > 0) {
  // Positive
} else {
  // Negative
}

versus

if (Math.sign(number) > 0) {
  // Positive
} else {
  // Negative
}

Indeed, if you're just checking the boolean status, then I'd just use the comparative operator instead of using Math.sign. But where Math.sign shines is it returns a number value. This means you can do calculations.

const number = 5;

number > 0; // true
Math.sign(number); // 1

Solving an Algorithm Challenge with Math.sign

So it allows me to solve this algorithm challenge: "Reverse an Integer"

Input: 123;
Output: 321;

Input: -123;
Output: -321;
function reverseInteger(num) {
  const numArray = Math.abs(num) // Get the absolute value of our number > 321
    .toString() // Convert our number to a string > '321'
    .split('') // Convert our string of numbers to an array > [3, 2, 1]
    .reverse() // Reverse our array of numbers > [1, 2, 3]
    .join(''); // Convert our array back to a string > 123

  const sign = Math.sign(num); // -1
  return numArray * sign;
  // Multiply our reverse string with the sign will produce the correct reverse number
}

reverseInteger(-321); // -123

This algorithm question is from Leetcode's "Reverse an Integer". I edited the requirement of the question to simplify our demonstration. To see the actual solution, here's one from @loia5tqd001 .

This is actually when I first discovered this function. That's why I love looking at other people's solutions. It's always interesting to how other people solve something. Even if the solution is bad, I read those too, cause it teaches me what to avoid 😉. No knowledge is wasted 💪. It's all expanding my toolkit. Just like those learning machines, the more data you feed, the better it gets. I think my brain is like that too. I need to see a lot of solutions in order for me to get better 😄

That's why in a lot of my tidbits, I cover the different ways of solving something. Because there is never a BEST function. The best way is always dependant on the situation. The larger your toolkit, the higher chance you will find the best way 👍

Negative Zero

So you may notice that Math.sign returns a negative zero:

Math.sign(0); // 0
Math.sign(-0); // -0

And your next question is, what the heck is this negative zero 🤨. Kyle Simpson of "You Don't Know JS" explains it the best:

Now, why do we need a negative zero, besides academic trivia?

There are certain applications where developers use the magnitude of a value to represent one piece of information (like speed of movement per animation frame) and the sign of that number to represent another piece of information (like the direction of that movement).

In those applications, as one example, if a variable arrives at zero and it loses its sign, then you would lose the information of what direction it was moving in before it arrived at zero. Preserving the sign of the zero prevents potentially unwanted information loss.

YDKJS - Type & Grammer - Zeros

Math.sign Browser Support

Support is great for all modern browsers. Unfortunately Internet Explorers is too hip to play with the rest of the class. So no support there.

Browser
Chrome
Firefox
Safari
Edge
Internet Explorer

MDN Browser Compatibility Chart

Code Tidbit for IE

But no worries, here is an alternative code snippet for you. This will work on Internet Explorer and older browsers 👍

const positive = 5;
const negative = -5;
const zero = 0;

positive === 0 ? positive : positive > 0 ? 1 : -1; // 1
negative === 0 ? negative : negative > 0 ? 1 : -1; // -1
zero === 0 ? zero : zero > 0 ? 1 : -1; // 0

Math.sign Polyfill

Or keep using Math.sign and just add this Polyfill from MDN

if (!Math.sign) {
  Math.sign = function(x) {
    return (x > 0) - (x < 0) || +x;
  };
}

Resources

Thanks for reading ❤
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Discussion (20)

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fluffynuts profile image
Davyd McColl • Edited on

slight error:

Math.sign either returns 1 or -1, so:

if (Math.sign(number)) {
  // always run
} else {
  // never run
}

did you perhaps mean if (Math.sign(number) > 0) ? in which case if (number > 0) is more concise.

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samanthaming profile image
Samantha Ming Author

OH good catch, let me fix it! Thanks for letting me know 😱

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dharanr profile image
Dharan

Its crazy to know, that even a positive / negative one line execution needs this much knowledge LoL

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samanthaming profile image
Samantha Ming Author

hahahaha 😂Tip of the iceberg...always more to know and more to learn when it comes to javascript 😆

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pony profile image
Pony

Gotcha!

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basarozcan profile image
Başar Özcan

you always find a good topic, thank you

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samanthaming profile image
Samantha Ming Author • Edited on

thank you! i just pick topics that i want to learn, luckily this one resonates with you 😄And if there is a topic you want me to cover, please let me know! 👏

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hafizpustice05 profile image
hafizpustice05

Great content.

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Samantha Ming Author

Thanks! Glad you liked the article 😄

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aralroca profile image
Aral Roca

+10 years with JavaScript and I never listen about negative 0! But make sense...

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samanthaming profile image
Samantha Ming Author

I only learned the negative 0 when i wrote this article. It will be interesting to see when i can actually apply it. I'll make sure to share when that happens 😊

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Lara Durrant

Thanks for the article! It’s always a good reminder that there’s probably a built in JavaScript method for pretty much anything you wanna do! :)

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Samantha Ming Author

Totally! JS is a big toolbox and continues to add more tools for us to use, which is awesome. Thanks for reading my article 👏

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somedood profile image
Basti Ortiz

You learn something new everyday. Thanks for sharing!

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Samantha Ming Author

I like that -- keep that student mindset so we can continue to learn and grow everyday! Thanks for reading 👏

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Benny Powers 🇮🇱🇨🇦

Love your posts, Samantha, keep them coming

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Samantha Ming Author

you bet! thanks for the encouragement benny 😊

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Nadia Guarracino • Edited on

Checks browser support
Be happy that is wide supported
Read the IE line
Laugh with sadness

By the way, cool tip!

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samanthaming profile image
Samantha Ming Author

Laugh with sadness -- a perfect description of IE 😵

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miteshkamat27 profile image
Mitesh Kamat

Good to use !!