How many numbers in the given array are less/equal to the given value using the percentile formula in Javascript?

In this article, you will understand how to calculate the percentile of a given value in an array using JavaScript. The percentile tells us what percentage of numbers in the array are less than or equal to a specific value.

Percentile Formula

We use the following formula to calculate the percentile:

Percentile = (n/N) * 100

Where:

• n = count of values less than or equal to the given value
• N = total number of values in the array

For values equal to our target, we typically count them as 0.5 to get a more accurate percentile calculation.

Method 1: Using for...in Loop

This approach uses a for-loop to iterate through the array and count values less than or equal to the target value.

const calculatePercentile = (inputArray, val) => {
   let count = 0;   
   for (let i in inputArray) {
      count += (inputArray[i] < val ? 1 : 0) + (inputArray[i] === val ? 0.5 : 0);
   }   
   let percentile = (count / inputArray.length) * 100;
   console.log("The percentile value is:", percentile);
   return percentile;
};   

const inputArray = [3, 4, 5, 6, 7, 8];
console.log("The array is:", inputArray);
console.log("Target value: 6");

calculatePercentile(inputArray, 6);

The array is: [ 3, 4, 5, 6, 7, 8 ]
Target value: 6
The percentile value is: 58.333333333333336

Method 2: Using reduce() Method

This approach uses the reduce() method to count values and calculate the percentile in a more functional programming style.

const calculatePercentile = (inputArray, inputValue) =>
   (100 * 
    inputArray.reduce(
       (count, value) => count + (value < inputValue ? 1 : 0) + (value === inputValue ? 0.5 : 0),
       0
    )) / inputArray.length;

const inputArray = [3, 4, 5, 6, 7, 8];
console.log("The array is:", inputArray);

let inputValue = 6;
console.log("Target value:", inputValue);

console.log("The percentile value is:", calculatePercentile(inputArray, inputValue));

The array is: [ 3, 4, 5, 6, 7, 8 ]
Target value: 6
The percentile value is: 58.333333333333336

How It Works

Both methods follow the same logic:

1. Count values less than the target (full count = 1)
2. Count values equal to the target (half count = 0.5)
3. Apply the percentile formula: (count / total) × 100

In our example with array [3, 4, 5, 6, 7, 8] and target value 6:

• Values less than 6: [3, 4, 5] = 3 values
• Values equal to 6: [6] = 1 value × 0.5 = 0.5
• Total count: 3 + 0.5 = 3.5
• Percentile: (3.5 / 6) × 100 = 58.33%

Comparison

Conclusion

Both methods effectively calculate percentiles using the standard formula. The for-loop approach is more explicit and readable, while the reduce() method offers a more functional programming style. Choose based on your coding preferences and team standards.