gistfile1.js

// Truncate a string to the closest word
String.prototype.truncateToWord = function(length) {
  return this
    .slice(0, length + 1)
    .split(/\s+/)
    .slice(0, -1)
    .join(" ");
};

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

//              v
"cut that shit out now!".truncateToWord(15); // "cut that shit"

//            v
"cut that shit out now!".truncateToWord(13); // "cut that shit"

//         v
"cut that shit out now!".truncateToWord(10); // "cut that"

gistfile2.js

// From @tr0gd0rr (Ken Snyder)
// Untested
String.prototype.truncateToWord = function(length) {
  return this.match(new RegExp('^(.{0,'+(+length)+'})(\\b|$)'))[1]
    .replace(/\s+$/,''); // ghetto quick-fix to remove trailing white space
};

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

//              v
"cut that shit out now!".truncateToWord(15); // "cut that shit"

//            v
"cut that shit out now!".truncateToWord(13); // "cut that shit"

//         v
"cut that shit out now!".truncateToWord(10); // "cut that"