How can I round a number in JavaScript? .toFixed() returns a string?

Question

Am I missing something here?

var someNumber = 123.456;
someNumber = someNumber.toFixed(2);
alert(typeof(someNumber));
//alerts string

Why does .toFixed() return a string?

I want to round the number to 2 decimal digits.

Answer 1

Number.prototype.toFixed is a function designed to format a number before printing it out. It's from the family of toString, toExponential and toPrecision.

To round a number, you would do this:

someNumber = 42.008;
someNumber = Math.round( someNumber * 1e2 ) / 1e2;
someNumber === 42.01;

// if you need 3 digits, replace 1e2 with 1e3 etc.
// or just copypaste this function to your code:

function toFixedNumber(num, digits, base){
  var pow = Math.pow(base||10, digits);
  return Math.round(num*pow) / pow;
}

.

Or if you want a “native-like” function, you can extend the prototype:

Number.prototype.toFixedNumber = function(digits, base){
  var pow = Math.pow(base||10, digits);
  return Math.round(this*pow) / pow;
}

someNumber = 42.008;
someNumber = someNumber.toFixedNumber(2);
someNumber === 42.01;


//or even hexadecimal

someNumber = 0xAF309/256  //which is af3.09
someNumber = someNumber.toFixedNumber(1, 16);
someNumber.toString(16) === "af3.1";

However, bear in mind that polluting the prototype is considered bad when you're writing a module, as modules shouldn't have any side effects. So, for a module, use the first function.