TypeScript – JavaScript: round number by decimal pecision

If we want to round a number at most with 2 decimal places, a common use is the follow:

var number = Use Math.round(num * 100) / 100;

While this will work for most cases, it will not work for 1.005 which will end up coming out to be 1 instead of 1.01, so it’s a wrong way to round a number.

Starting from the example proposed over the precisionRound that I found over MDN (that event for 1.005 returs 1 and not 1.01), I write a custom precisionRound that manage a random precision number and for 1.005 returns 1.01.

This is the function:

function precisionRound(number, precision)
{
  if(precision < 0)
  {
    var factor = Math.pow(10, precision);
    return Math.round(number * factor) / factor;
  }
  else
    return +(Math.round(number + "e+"+precision) + 
      "e-" + precision);
}

console.log(precisionRound(1234.5678, 1));  // output: 1234.6
console.log(precisionRound(1234.5678, -1)); // output: 1230
console.log(precisionRound(1.005, 2));      // output: 1.01
console.log(precisionRound(1.0005, 2));     // output: 1
console.log(precisionRound(1.0005, 3));     // output: 1.001
console.log(precisionRound(1.0005, 4));     // output: 1.0005

For TypeScript:

public static precisionRound(number: number, precision: number)
{
  if (precision < 0)
  {
    let factor = Math.pow(10, precision);
    return Math.round(number * factor) / factor;
  }
  else
    return +(Math.round(Number(number + "e+" + precision)) +
      "e-" + precision);
}