Javascript implementation of a Shannon entropy calculation in bits per symbol

entropy-in-short.js

// Shannon entropy
const entropy = str => {
  return [...new Set(str)]
    .map(chr => {
      return str.match(new RegExp(chr, 'g')).length;
    })
    .reduce((sum, frequency) => {
      let p = frequency / str.length;
      return sum + p * Math.log2(1 / p);
    }, 0);
};

entropy.js

// entropy.js MIT License © 2014 James Abney http://github.com/jabney
// ES6 portation MIT License © 2017 Peter Seprus http://github.com/ppseprus

// Calculate the Shannon entropy of a string in bits per symbol.
(function (shannon) {
    'use strict';

    // Create an array of character frequencies.
    const getFrequencies = str => {
        let dict = new Set(str);
        return [...dict].map(chr => {
            return str.match(new RegExp(chr, 'g')).length;
        });
    };

    // Measure the entropy of a string in bits per symbol.
    shannon.entropy = str => getFrequencies(str)
        .reduce((sum, frequency) => {
            let p = frequency / str.length;
            return sum - (p * Math.log(p) / Math.log(2));
        }, 0);

    // Measure the entropy of a string in total bits.
    shannon.bits = str => shannon.entropy(str) * str.length;

    // Log the entropy of a string to the console.
    shannon.log = str => console.log(`Entropy of "${str}" in bits per symbol:`, shannon.entropy(str));

})(window.shannon = window.shannon || Object.create(null));
 
shannon.log('1223334444');          // 1.8464393446710154
shannon.log('0');                   // 0
shannon.log('01');                  // 1
shannon.log('0123');                // 2
shannon.log('01234567');            // 3
shannon.log('0123456789abcdef');    // 4

Amazing, thanks for sharing!

new RegExp(chr, 'g') will fail with unescaped characters like +*{} etc. This may be circumvented by using new RegExp('[' + chr + ']', 'g'). Or by not using a RegExp at all:

function frequencies(str) {
  const res = {};
  str = [...str];
  for (let chr of str) { res[chr] = (res[chr] ?? 0) + 1;  }
  return Object.values(res);
}

This is a fork from 2017, so that I'd have a copy... You should probably to to the original.