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)
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javascript
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If you want to pass the % as part of your function you should use the following alternative:
<script>
function fpercentStr(quantity, percentString)
{
var percent = new Number(percentString.replace("%", ""));
return fpercent(quantity, percent);
}
function fpercent(quantity, percent)
{
return quantity * percent / 100;
}
document.write("test 1: " + fpercent(10000, 35.873))
document.write("test 2: " + fpercentStr(10000, "35.873%"))
</script>
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.
var number = 10000;
var result = .358 * number;
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
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);
}
It may be a bit pedantic / redundant with its numeric casting, but here's a safe function to calculate percentage of a given number:
function getPerc(num, percent) {
return Number(num) - ((Number(percent) / 100) * Number(num));
}
// Usage: getPerc(10000, 25);
This is what I would do:
// num is your number
// amount is your percentage
function per(num, amount){
return num*amount/100;
}
...
<html goes here>
...
alert(per(10000, 35.8));
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:
amount and the percentamount and percent to integers using exponential notationThe 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.
Your percentage divided by 100 (to get the percentage between 0 and 1) times by the number
35.8/100*10000
Source: Stackoverflow.com