[removed] calculate x% of a number

Question

I am wondering how in javascript if i was given a number (say 10000) and then was given a percentage (say 35.8%)

how would I work out how much that is (eg 3580)

Answer 1

Best thing is to memorize balance equation in natural way.

Amount / Whole = Percentage / 100

---

usually You have one variable missing, in this case it is Amount

Amount / 10000 = 35.8 / 100

then you have high school math (proportion) to multiple outer from both sides and inner from both sides.

Amount * 100 = 358 000

Amount = 3580

It works the same in all languages and on paper. JavaScript is no exception.

Answer 2

I use two very useful JS functions: http://blog.bassta.bg/2013/05/rangetopercent-and-percenttorange/

function rangeToPercent(number, min, max){
   return ((number - min) / (max - min));
}

and

function percentToRange(percent, min, max) {
   return((max - min) * percent + min);
}

Answer 3

var result = (35.8 / 100) * 10000;

(Thank you jball for this change of order of operations. I didn't consider it).

Answer 4

Harder Way (learning purpose) :

var number = 150
var percent= 10
var result = 0
for (var index = 0; index < number; index++) {
   const calculate = index / number * 100
   if (calculate == percent) result += index
}
return result

Answer 5

Your percentage divided by 100 (to get the percentage between 0 and 1) times by the number

35.8/100*10000

Answer 6

In order to fully avoid floating point issues, the amount whose percent is being calculated and the percent itself need to be converted to integers. Here's how I resolved this:

function calculatePercent(amount, percent) {
    const amountDecimals = getNumberOfDecimals(amount);
    const percentDecimals = getNumberOfDecimals(percent);
    const amountAsInteger = Math.round(amount + `e${amountDecimals}`);
    const percentAsInteger = Math.round(percent + `e${percentDecimals}`);
    const precisionCorrection = `e-${amountDecimals + percentDecimals + 2}`;    // add 2 to scale by an additional 100 since the percentage supplied is 100x the actual multiple (e.g. 35.8% is passed as 35.8, but as a proper multiple is 0.358)

    return Number((amountAsInteger * percentAsInteger) + precisionCorrection);
}

function getNumberOfDecimals(number) {
    const decimals = parseFloat(number).toString().split('.')[1];

    if (decimals) {
        return decimals.length;
    }

    return 0;
}

calculatePercent(20.05, 10); // 2.005

As you can see, I:

1. Count the number of decimals in both the amount and the percent
2. Convert both amount and percent to integers using exponential notation
3. Calculate the exponential notation needed to determine the proper end value
4. Calculate the end value

The usage of exponential notation was inspired by Jack Moore's blog post. I'm sure my syntax could be shorter, but I wanted to be as explicit as possible in my usage of variable names and explaining each step.