kmp.js

function substring(txt, str) {
  var dfa = compile(str);
  var curr = 0;
  for(var i=0; i<txt.length; i++) {
    curr = next(dfa, curr, txt.charAt(i));
    if(curr === str.length)
      return true;
  }


  return false;
}

function compile(str) {
  var dfa = [[]];
  var chars = {};
  var prev = 0;
  for(var i=0; i<str.length; i++) {
    dfa[i] = {};

    if (i > 0) {
      for(ch in chars) {
        dfa[i][ch] = next(dfa, prev, ch);
      }
      prev = next(dfa, prev, str.charAt(i));
    }

    // Add to a Set of all unique characters
    chars[str.charAt(i)] = true;
    dfa[i][str.charAt(i)] = i + 1;

  }

  return dfa;
}

function next(dfa, curr, ch) {
  var next = dfa[curr][ch];

  if(!next) return 0;

  return next;
}

console.log(substring('ABABABABAC', 'ABABAC'));