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Trying to get some zip code expansion js code, I got the following:

function parse(string) {
    const numbers = [];
    for (const [, beginStr, endStr] of string.matchAll(/(\d+)(?:-(\d+))?/g)) {
        const [begin, end] = [beginStr, endStr].map(Number);
        numbers.push(begin.toString().padStart(5, "0"));
        if (endStr !== undefined) {
            for (let num = begin + 1; num <= end; num++) {
              
              numbers.push(num.toString().padStart(5, "0"));
          }
        }
    }
    return numbers;
}

There are a couple of things I don't understand. First off, why is there a comma when declaring beginStr and endStr in the for .. of loop? Two, the regex has a ?: that is explained like "? matches the previous token between zero and one times, as many times as possible, giving back as needed", but there is no previous token and the colon is not explained. Does anybody know how this works?

Thanks, Raf

user167582
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1 Answers1

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The matchAll returns an Iterator with a value of [ZIP range, ZIP start, ZIP end]. Since the code does not need the ZIP range, you can skip that value when destructuring the array. So it's basically

const [
  , // ZIP range
  beginStr, // ZIP start
  endStr, // ZIP end
]

As for the ?: What is a non-capturing group in regular expressions?

dork
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