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(Note: Remarks in my first review about handling special cases and the returning a value still apply.)

(Note: Remarks in my first review about handling special cases and the returning a value still apply.)

function transposeString(str) {
 if (str == null) return str;
 var words = str.split(' ');
 var maxWordLength = _.max(_.pluck(words, 'length'));
 return _.range(maxWordLength).map(
 // Generate a line of output using the nth character for each word
 function outputLineUsingNthChar(n) {
 return words.map(
 // Extract the nth character of a word, space-padded if needed
 function nthCharWithSpacePadding(word) {
 return n < word.length ? word.charAt(n) : ' ';
 }
 ).join('');
 }
 ).join("\n");
}

function transposeString(str) {
 if (str == null) return str;
 var words = str.split(' ');
 var maxWordLength = _.max(_.pluck(words, 'length'));
 return _.range(maxWordLength).map(
 // Generate a line of output using the nth character of each word
 function outputLineUsingNthChar(n) {
 return words.map(
 // Extract the nth character of a word, space-padded if needed
 function nthCharWithSpacePadding(word) {
 return n < word.length ? word.charAt(n) : ' ';
 }
 ).join('');
 }
 ).join("\n");
}

I would like to point out that your solution feels a little like functional programming, though not quite. What if you go all the way with functional programming? Your solution was already quite close; you just need to replace .forEach() with .map(). Here's how it could look:

Your solution feels a little like functional programming, though not quite. What if you go all the way with functional programming? Your solution was already quite close; you just need to replace .forEach() with .map(). Here's how it could look: