JavaScript Huffman Encoding algorithm

huffman.js

const getFrequency = text => {
  let freqs = [];

  for (let i = 0; i < text.length; i++) {
    const letter = text[i];
    const letterIndex = freqs.findIndex(l => l.symbol === letter);

    if (letterIndex > -1) {
      freqs[letterIndex].weight++;
    } else {
      freqs.push({
        symbol: letter,
        weight: 1,
      });
    }
  }

  return freqs.sort((a, b) => a.weight - b.weight);
};

const buildTree = letters => {
  let tree = letters;

  while (tree.length > 1) {
    const [s1, s2, ...rest] = tree;
    const newWeight = s1.weight + s2.weight;

    tree = rest;
    tree.push({
      symbol: s1.symbol + s2.symbol,
      weight: newWeight,
      0: s1,
      1: s2,
    });
    tree.sort((a, b) => a.weight - b.weight);
  }

  return tree[0];
};

const buildSymbolCodes = (node, side = '', code = '') => {
  const { symbol } = node;
  const newCode = code + side;

  if (symbol.length === 1) {
    return [{ symbol, code: newCode }];
  }

  return [
    ...buildSymbolCodes(node[0], 0, newCode),
    ...buildSymbolCodes(node[1], 1, newCode),
  ];
};

const encode = (string, codes) => {
  codes.forEach(symbol => {
    string = string.replace(new RegExp(symbol.symbol, 'g'), symbol.code);
  });

  return string;
};

const decode = (string, tree) => {
  let original = '';
  let node = tree;

  for (let i = 0; i < string.length; i++) {
    const digit = string[i];
    let letter = '';
    const leaf = node[digit];

    if (leaf.symbol.length === 1) {
      letter += leaf.symbol;
      node = tree;
    } else {
      node = leaf;
    }

    original += letter;
  }

  return original;
};

const [input] = process.argv.slice(2);
const inputs = [
  'ABRACADABRA',
  'MISSISSIPPI RIVER',
  'A_DEAD_DAD_CEDED_A_BAD_BABE_A_BEADED_ABACA_BED',
];
const random = inputs[Math.floor(Math.random() * inputs.length)];

const start = string => {
  console.log('original: ', string);

  const freq = getFrequency(string);
  const tree = buildTree(freq);
  const codes = buildSymbolCodes(tree);

  const code = encode(string, codes);
  console.log('encoded: ', code);

  console.log('decoded: ', decode(code, tree));
};

start(input || random);