Just wanted to point out that @200_success's review is excellent, but personally I would go for reduce:

function longestWord2(str){
    var words = str.replace(/[^A-Za-z\s]/g, "").split(" ");
    var longestWord = words.reduce( function( longestSoFar, currentWord ) {
      return currentWord.length > longestSoFar.length ? currentWord : longestSoFar;
    },"");
    return longestWord;
}

document.write( longestWord2( 'Code Review welcomes you' ) );

Using reduce will access each word only once. It will also return the first word of the largest length if there are multiple words with the same largest length;

Just wanted to point out that @200_success's review is excellent, but personally I would go for reduce:

    function longestWord2(str){
        var words = str.replace(/[^A-Za-z\s]/g, "").split(" ");
        var longestWord = words.reduce( function( longestSoFar, currentWord ) {
          return currentWord.length > longestSoFar.length ? currentWord : longestSoFar;
        },"");
        return longestWord;
    }
    
    document.write( longestWord2( 'Code Review welcomes you' ) );

Using reduce will access each word only once. It will also return the first word of the largest length if there are multiple words with the same largest length;