https://techdevguide.withgoogle.com/paths/foundational/find-longest-word-in-dictionary-that-subsequence-of-given-string#code-challenge

subsequence.js

"use strict";

const s = "abppplee";
const D = ["able", "ale", "apple", "bale", "kangaroo"];

(() => {
  // console.log(inline(s, D[4]));
  const letterPositions = toCharMap(s);

  D.sort((a, b) => {
    return b.length - a.length;
  });

  for (let i = 0; i < D.length; i++) {
    const word = D[i];
    let pos = 0;
    let found = false;
    for (let j = 0; j < word.length; j++) {
      const letter = word.charAt(j);
      if (!letterPositions[letter]) {
        break;
      }
      const possiblePositions = letterPositions[letter].map(p => p >= pos);
      if (possiblePositions.length == 0) {
        break;
      }
      pos = possiblePositions[0] + 1;
      if (j == word.length - 1) {
        found = true;
      }
    }
    if (found) {
      console.log(word);
      break;
    }
  }
})();

function toCharMap(str) {
  const charMap = {};
  for (let i = 0; i < str.length; i++) {
    const c = str.charAt(i);
    if (!charMap[c]) {
      charMap[c] = [i];
    } else {
      charMap[c].push(i);
    }
  }

  return charMap;
}

function inline(str1, str2) {
  let x = 0;
  for (let i = 0; i < str2.length; i++) {
    const c = str2.charAt(i);
    const p = str1.indexOf(c);
    if (p === -1 || p < x) {
      return -1;
    }

    x++;
  }

  return x;
}