[javascript] JavaScript math, round to two decimal places

I have the following JavaScript syntax:

var discount = Math.round(100 - (price / listprice) * 100);

This rounds up to the whole number. How can I return the result with two decimal places?

The best and simple solution I found is

function round(value, decimals) {
 return Number(Math.round(value+'e'+decimals)+'e-'+decimals);
}   
round(1.005, 2); // 1.01

I think the best way I've seen it done is multiplying by 10 to the power of the number of digits, then doing a Math.round, then finally dividing by 10 to the power of digits. Here is a simple function I use in typescript:

function roundToXDigits(value: number, digits: number) {
    value = value * Math.pow(10, digits);
    value = Math.round(value);
    value = value / Math.pow(10, digits);
    return value;
}

Or plain javascript:

function roundToXDigits(value, digits) {
    if(!digits){
        digits = 2;
    }
    value = value * Math.pow(10, digits);
    value = Math.round(value);
    value = value / Math.pow(10, digits);
    return value;
}

To handle rounding to any number of decimal places, a function with 2 lines of code will suffice for most needs. Here's some sample code to play with.

    var testNum = 134.9567654;
    var decPl = 2;
    var testRes = roundDec(testNum,decPl);  
    alert (testNum + ' rounded to ' + decPl + ' decimal places is ' + testRes);

    function roundDec(nbr,dec_places){
        var mult = Math.pow(10,dec_places);
        return Math.round(nbr * mult) / mult;
    }

A small variation on the accepted answer. toFixed(2) returns a string, and you will always get two decimal places. These might be zeros. If you would like to suppress final zero(s), simply do this:

var discount = + ((price / listprice).toFixed(2));

Edited: I've just discovered what seems to be a bug in Firefox 35.0.1, which means that the above may give NaN with some values.
I've changed my code to

var discount = Math.round(price / listprice * 100) / 100;

This gives a number with up to two decimal places. If you wanted three, you would multiply and divide by 1000, and so on.
The OP wants two decimal places always, but if toFixed() is broken in Firefox it needs fixing first.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1134388

The functions Math.round() and .toFixed() is meant to round to the nearest integer. You'll get incorrect results when dealing with decimals and using the "multiply and divide" method for Math.round() or parameter for .toFixed(). For example, if you try to round 1.005 using Math.round(1.005 * 100) / 100 then you'll get the result of 1, and 1.00 using .toFixed(2) instead of getting the correct answer of 1.01.

You can use following to solve this issue:

Number(Math.round(100 - (price / listprice) * 100 + 'e2') + 'e-2');

Add .toFixed(2) to get the two decimal places you wanted.

Number(Math.round(100 - (price / listprice) * 100 + 'e2') + 'e-2').toFixed(2);

You could make a function that will handle the rounding for you:

function round(value, decimals) {
    return Number(Math.round(value + 'e' + decimals) + 'e-' + decimals);
}

Example: https://jsfiddle.net/k5tpq3pd/36/

Alternativ

You can add a round function to Number using prototype. I would not suggest adding .toFixed() here as it would return a string instead of number.

Number.prototype.round = function(decimals) {
    return Number((Math.round(this + "e" + decimals)  + "e-" + decimals));
}

and use it like this:

var numberToRound = 100 - (price / listprice) * 100;
numberToRound.round(2);
numberToRound.round(2).toFixed(2); //Converts it to string with two decimals

Example https://jsfiddle.net/k5tpq3pd/35/

Source: http://www.jacklmoore.com/notes/rounding-in-javascript/

try using discount.toFixed(2);

    function round(num,dec)
    {
      num = Math.round(num+'e'+dec)
      return Number(num+'e-'+dec)
    }
      //Round to a decimal of your choosing:
    round(1.3453,2)

Here is a working example

var value=200.2365455;
result=Math.round(value*100)/100    //result will be 200.24

To get the result with two decimals, you can do like this :

var discount = Math.round((100 - (price / listprice) * 100) * 100) / 100;

The value to be rounded is multiplied by 100 to keep the first two digits, then we divide by 100 to get the actual result.

Fastest Way - faster than toFixed():

TWO DECIMALS

x      = .123456
result = Math.round(x * 100) / 100  // result .12

THREE DECIMALS

x      = .123456
result = Math.round(x * 1000) / 1000      // result .123

If you use a unary plus to convert a string to a number as documented on MDN.

For example:+discount.toFixed(2)