re: Refactoring the Worst Code I’ve Ever Written VIEW POST


But nested loops can have significant negative performance impacts. This method focuses on each layer of categorization separately, eliminating extraneous looping and improving performance.

It's awesome that you've got an eye out for performance - that's always great to see. You are right that nested loops can be problematic, but keep in mind that the cost of the work done in the loop is the main factor.

In this case it's actually 6x faster to build your structure using nested loops, since the cost of creating empty arrays is tiny compared to the overhead of all the other calls in your write-up. I'd argue that the nested loop version is easier to understand too:

const types = ['in', 'out'];
const years = [2000, 2005, 2010, 2015, 2016, 2017];
const regions = ['Africa', 'Americas', 'Asia', 'Europe', 'Oceania'];

var dataset = {};

for (let typ of types) {
  dataset[typ] = {};

  for (let year of years) {
    dataset[typ][year] = {};

    for (let region of regions) {
      dataset[typ][year][region] = [];

As the number of values in types, years and regions increases, the nested loop implementation remains 6x faster as the main cost is running the rest of the code, not creating the array.

It's cool that you know how to reduce iterations like this, but nested loops aren't automatically a bad thing 🙂


Thanks for the comment! I'm aware that nested loops aren't always a bad thing, that's why I included the word "can" in the part of my article you quoted. :)

In this case, you're right, the nested loop is faster, so I stand corrected on the claim that the solution I proposed is better for performance. Most "best practice" articles/guides I've read have all suggested to stay away from nested loops when possible, so I opted to highlight a different method. Of course it's all context specific, so the "best" way is going to be variable depending on the problem being solved.

In any case, I've updated my article with a link to your comment so others can see it as well. Thanks!

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