This seems to be a popular interview question that I stumbled upon in an Anki deck
const compress = str => {
//build a character frequency map
const freqM = [...str].reduce((acc, val) => {
acc[val] = acc[val] + 1 || 1;
return acc;
}, {});
//return str if length <= unique characters * 2 (ie, 'A' vs 'A1')
if (str.length <= Object.keys(freqM).length * 2) return str;
//return the frequency map as a string otherwise
return Object.entries(freqM).flat().join('');
}
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