gistfile1.js

// Truncate a string to the closest word
String.prototype.truncateToWord = function(len) {
  return (len = Number(len)) >= 0 ?
    this.match(new RegExp('^([\\s\\S]{0,' + len + '})(:?\\s|$)'))[1] :
    undefined;
};

// Examples
// The "v" marks the first character out of bounds.

//    v  
"A rambunctious child jumps. Does he eat pavement?".truncateToWord(5); // "A"

//                            v
"A rambunctious child jumps. Does he eat pavement?".truncateToWord("31"); // "A rambunctious child jumps."

"".truncateToWord(10); // ""

"text".truncateToWord(0); // ""

"text".truncateToWord(-1); // undefined

"text".truncateToWord("a"); // undefined

//       v
"line1\nline2".truncateToWord(8); // "line1"

//             v
"line1\nline2".truncateToWord(14); // "line1\nline2"