# Detect the intersection of multiple ranges in JavaScript

Published on Monday, June 20, 2022

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Recently, while working on a feature for Octo, I had to figure out how to test the intersection of multiple ranges using JavaScript. Specifically, I needed to come up with a performant solution to detect when any text-selection ranges intersected with specific syntax node ranges in the editor. This was a fun problem to work on, so I hope you like the solution.

### How to compare ranges

A range can reasonably be represented by a starting point and an ending point. For the purpose of this post, we will assume the starting point is the minimum and the ending point is the maximum.

We will know multiple ranges are intersecting when the maximal minimum (the largest starting point) of the ranges is less than or equal to the minimal maximum (the smallest ending point). We can make this abstract concept a little more concrete by plotting two ranges on a number line.

``   [0, 1, 2, 3, 4]//--0--1--2--3--4--5--6--7--8--9--->         [2, 3, 4, 5, 6]``

The minimums of the ranges are `[0, 2]`, and the maximums of the ranges are `[4, 6]`. The maximal value of the minimums is `2`, and the minimal value of the maximums is `4`. Since `2` is less than or equal to `4`, we can safely say the two ranges intersect.

## Detect the intersection of all ranges

For these examples, we will use the following `range` function.

``type NumberRange = { min: number, max: number }const range = (min: number, max: number): NumberRange => {  return { min, max }}``

The method we learned above works for any number of ranges as along as you want to assert that all of the ranges intersect (more on that later). We can implement the logic from the previous example as a function that accepts any number of ranges.

``const allIntersect = (...ranges: NumberRange[]) => {  // Create separate arrays for the min and max values.  const minimums = ranges.map(r => r.min)  const maximums = ranges.map(r => r.max)  // Extract the comparands.  const maximalMinimum = Math.max(...minimums)  const minimalMaximum = Math.min(...maximums)  return maximalMinimum <= minimalMaximum}``

Then, we can use this function to detect whether the given ranges are intersecting.

``allIntersect(range(0, 4), range(2, 6), range(4, 8))// trueallIntersect(range(0, 4), range(2, 6), range(10, 14))// false``

## Detect the intersection of some ranges

The implementation above works well for checking whether all of the ranges intersect, but it is sometimes necessary to check whether some of the ranges intersect. We can create another function for that purpose which builds upon our original solution.

``const someIntersect = (...ranges: NumberRange[]) => {  return ranges.some((range1, index1) => {    // Compare all ranges to all other ranges.    return ranges.some((range2, index2) => {      // Do not compare a range to itself.      if (index1 === index2) {        return false      }      return allIntersect(range1, range2)    })  })}``
``someIntersect(range(0, 4), range(2, 6), range(4, 8))// truesomeIntersect(range(0, 4), range(2, 6), range(10, 14))// truesomeIntersect(range(0, 4), range(10, 14))// false``

This one might be a bit niche, but I would love to hear about your own use case. If you found this post helpful, you want to know more, you have a question, or you just want to say hi, please leave a comment down below!

Thanks for reading, and happy coding. ✌️

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