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

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.

Why not * the result by 1 i.e

someNumber.toFixed(2) * 1

May be too late to answer but you can multiple the output with 1 to convert to number again, here is an example.

const x1 = 1211.1212121;
const x2 = x1.toFixed(2)*1;
console.log(typeof(x2));

For others like me that happen upon this very old question, a modern solution:

const roundValue = (num, decimals = 2) => {
    let scaling = 10 ** decimals;
    return Math.round((num + Number.EPSILON) * scaling) / scaling;
}

ref: https://stackoverflow.com/a/11832950

What would you expect it to return when it's supposed to format a number ? If you have a number you can't pretty much do anything with it because e.g.2 == 2.0 == 2.00 etc. so it has to be a string.

You can simply use a '+' to convert the result to a number.

var x = 22.032423;
x = +x.toFixed(2); // x = 22.03

I've solved this problem by changing this:

someNumber = someNumber.toFixed(2)

...to this:

someNumber = +someNumber.toFixed(2);

However this will convert the number to a string and parse it again, which will have a significant impact on performance. If you care about performance or type safety, check the the other answers as well.

Here's a slightly more functional version of the answer m93a provided.

const toFixedNumber = (toFixTo = 2, base = 10) => num => {
  const pow = Math.pow(base, toFixTo)
  return +(Math.round(num * pow) / pow)
}

const oneNumber = 10.12323223

const result1 = toFixedNumber(2)(oneNumber) // 10.12
const result2 = toFixedNumber(3)(oneNumber) // 10.123

// or using pipeline-operator
const result3 = oneNumber |> toFixedNumber(2) // 10.12

Of course it returns a string. If you wanted to round the numeric variable you'd use Math.round() instead. The point of toFixed is to format the number with a fixed number of decimal places for display to the user.

Because its primary use is displaying numbers? If you want to round numbers, use Math.round() with apropriate factors.

You should use it like below.

var someNumber: number = 0.000000;
someNumber = Number(someNumber.toFixed(2))

I solved it with converting it back to number using JavaScript's Number() function

var x = 2.2873424;
x = Number(x.toFixed(2));

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.

To supply an example of why it has to be a string:

If you format 1.toFixed(2) you would get '1.00'.

This is not the same as 1, as 1 does not have 2 decimals.

I know JavaScript isn't exactly a performance language, but chances are you'd get better performance for a rounding if you use something like: roundedValue = Math.round(value * 100) * 0.01

Why not use parseFloat?

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