Rounding of negative numbers in Javascript

Question

We have come across a problem with Math.round() in JavaScript. The problem is that this function doesn't round correctly for negative numbers. For example :

1.5 ~= 2

0.5 ~= 1

-0.5 ~= 0 // Wrong

-1.5 ~= -1 // Wrong

And this is not correct according to arithmetic rounding. The correct numbers for -0.5 should be -1 and -1.5 should be -2.

Is there any standard way, to correctly round negative numbers in Javascript ?

Answer 1

Apply Math.round after converting to a positive number and finally roll back the sign. Where you can use Math.sign method to get the sign from the number and Math.abs to get the absolute of the number.

console.log(
  Math.sign(num) * Math.round(Math.sign(num) * num),
  // or
  Math.sign(num) * Math.round(Math.abs(num))
)

var nums = [-0.5, 1.5, 3, 3.6, -4.8, -1.3];

nums.forEach(function(num) {
  console.log(
    Math.sign(num) * Math.round(Math.sign(num) * num),
    Math.sign(num) * Math.round(Math.abs(num))
  )
});

Answer 2

You could save the sign and apply later, in ES5;

function round(v) {
    return (v >= 0 || -1) * Math.round(Math.abs(v));
}

console.log(round(1.5));  //  2
console.log(round(0.5));  //  1
console.log(round(-1.5)); // -2
console.log(round(-0.5)); // -1