Javascript: percentage difference between two values

I am using the following javascript code to display the difference in percentage between two values.

A = 11192;
B = 10474;

percDiff = (vars.A - vars.B) / vars.B * 100;

Which gives: 6.855069696391064

Then

if (percDiff > 20) {
  //dosomething
} else {
  //dosomething
}

The problem is:

If value B is higher than value A then i get a NaN, for instance;

How can I overcome this issue? I thought about using Math.abs()

Any suggestions?

Answer

I think you can use this formula to determine the relative difference (percentage) between 2 values:

var percDiff =  100 * Math.abs( (A - B) / ( (A+B)/2 ) );

Here’s a utility method:

function relDiff(a, b) {
 return  100 * Math.abs( ( a - b ) / ( (a+b)/2 ) );
}
// example
relDiff(11240, 11192); //=> 0.42796005706134094

Note for people thinking the answer provides a wrong result: this question/answer concerns percentage difference¹, not percentage change².

Here’s a snippet to demonstrate the difference (pun intended;)

const relativePercentageDifference = (a, b) => 
  Math.abs( ( ( a - b ) / ( ( a + b ) / 2 ) ) * 100 );
const percentageChange = (a, b) => ( b / a * 100 ) - 100;
console.log(`relative % difference 2000 - 1000: ${
  relativePercentageDifference(2000, 1000).toFixed(2)}%`);
console.log(`% change 2000 - 1000: ${
  percentageChange(2000, 1000).toFixed(2)}%`);
console.log(`relative % difference 1000 - 2000: ${
  relativePercentageDifference(1000, 2000).toFixed(2)}%`);
console.log(`% change 1000 - 2000: ${
  percentageChange(1000, 2000).toFixed(2)}%`);

JsFiddle

¹ The percentual difference between two values divided by the average of the two values.

² The percentual difference (in-/decrease) between two numbers divided by the original number.