var removeUselessWords = function(txt) {
var uselessWordsArray =
[
"a", "at", "be", "can", "cant", "could", "couldnt",
"do", "does", "how", "i", "in", "is", "many", "much", "of",
"on", "or", "should", "shouldnt", "so", "such", "the",
"them", "they", "to", "us", "we", "what", "who", "why",
"with", "wont", "would", "wouldnt", "you"
];
var expStr = uselessWordsArray.join("|");
return txt.replace(new RegExp('(' + expStr + ')', 'gi'), ' ')
.replace(/s{2,}/g, ' ');
}
var str = "The person is going on a walk in the park. The person told us to do what we need to do in the park";
console.log(removeUselessWords(str));