How to round a number in JavaScript without using toFixed()

Question

I am trying to round numbers to a certain decimal place, the expected API would be something like this:

const rounded = Math.round(1.6180339, 5);

or

const rounded = new Number(1.6180339).round(5);

but those APIs don't seem to exist. I have this which appears to work as is:

const [
  e,
  π,
  φ
] = [
  2.71828182845904,
  3.14159265358979,
  1.61803398874989,
];


console.log(
  Number(e.toFixed(5)) === 2.71828  // true
);

console.log(
  Number(e.toFixed(3)) === 2.718    // true
);

console.log(
  Number(e.toFixed(3)) === 2.7182   // false
);

console.log(
  Number(e.toFixed(3)) === 2.71     // false
);

this works, but we have to use toFixed() which converts number to a string first. Is there a way to round a number directly without converting to a string?

Answer 1

Like mentioned in comments, floating point operations can be a pain with javascript. Anyway, you can still build a tool like this one that relies on powers of 10 to perform roundings and avoid string conversion step :

const Rounder = {
  floor(n, m) {
      return Math.floor(n * Math.pow(10, m)) / Math.pow(10, m);
  },
  ceil(n, m) {
      return Math.ceil(n * Math.pow(10, m)) / Math.pow(10, m);
  },
  round(n, m) {
      return Math.round(n * Math.pow(10, m)) / Math.pow(10, m);
  }
}

console.log(Rounder.ceil(7.1812828, 3));
console.log(Rounder.floor(7.1812828, 5));
console.log(Rounder.round(7.1812828, 2));
console.log(Rounder.round(0.11111111111, 8));
console.log(Rounder.ceil(0.11111111111, 8));