After staring at your code for some time,

• You are using Hungarian notation, that is not a JavaScript practice
  • Related to that, strArr probably should have been arrStr if the code was consistent
• It seems odd that you would return 0 when either word is not present, 0 would indicate that the words are neighbours. I would return -1 in that case.
• You seem to track too much state in acc, you can get away with just tracking the last matched word, the location of the last matched word and what the best distance has been thus far
• There are no comments whatsoever, I think your code could have used some

This is my functional approach:

const s = "dog cat hello cat dog dog hello cat dog world dog hello";
​
function smallestWordDistance(s, w1, w2) {
​
  function analyze(acc, word, index) {
​
    if (word == w1 || word == w2) {
​
      if (acc.word && acc.word != word) {
        let distance = index - acc.index - 1;
        acc.out = acc.out == -1 ? distance : Math.min(acc.out, distance);
      }
      acc.index = index;
      acc.word = word;
    }
    return acc;
  }
​
  return s.split(' ').reduce(analyze, { out: -1 }).out;
}
​
console.log(smallestWordDistance(s, "hello", "world"));

After staring at your code for some time,

• You are using Hungarian notation, that is not a JavaScript practice
  • Related to that, strArr probably should have been arrStr if the code was consistent
• It seems odd that you would return 0 when either word is not present, 0 would indicate that the words are neighbours. I would return -1 in that case.
• You seem to track too much state in acc, you can get away with just tracking the last matched word, the location of the last matched word and what the best distance has been thus far
• There are no comments whatsoever, I think your code could have used some

This is my functional approach:

const s = "dog cat hello cat dog dog hello cat dog world dog hello";
​
function smallestWordDistance(s, w1, w2) {
​
  function analyze(acc, word, index) {
​
    if (word == w1 || word == w2) {
​
      if (acc.word && acc.word != word) {
        const distance = index - acc.index - 1;
        acc.out = acc.out == -1 ? distance : Math.min(acc.out, distance);
      }
      acc.index = index;
      acc.word = word;
    }
    return acc;
  }
​
  return s.split(' ').reduce(analyze, { out: -1 }).out;
}
​
console.log(smallestWordDistance(s, "hello", "world"));