Coding Challenge, buffer problem

index.js

//Author: Michael Draper <mdraper@gmail.com>
//Justifies a string into a buffer.
// I ran it on Node v9.5.0 in ES2017 syntax.

const  isInt = (n) => {
    return n % 1 === 0;
 }
//Example: justify('The quick brown fox jumps over the lazy dog', 52)
const justify = (str, len) => {
    if(typeof(str) !== 'string'){
        throw new Error('cannot justify string: input is not a string');
    }
    if(typeof(len) !== 'number'){
        throw new Error('cannot justify string: len is not a number');
    }
    if(!isInt(len)){
        throw new Error('cannot justify string: len must be a whole number');
    }
    console.debug('len', len, 'str.length', str.length);
    if(len <= str.length) {
        throw new Error('cannot justify string: the specified length is too short to justify');
    }
    //Deconstruct the original string into its constiuent parts.

    const words = str.split(' '); // "The", "Quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"

    // Gotta have map/reduce, just because.. it's map/reduce!
    const charLen = words.map(w => w.length).reduce( (sum, w) => sum + w ); //Total length of all characters
                                                                            // In all words.

    if(len < charLen) {
        //TODO: Improve this message
        throw new Error("cannot justify string: input string is larger than the allowed buffer size");
    }

                     //52 - 34 = 18                
    const numSpaces = len - charLen; // Derives the number of total spaces which require allocation.

    //Require a minimum of 1 space between words
    if(numSpaces <= words.length - 1) {
        throw new Error('cannot justify string: the desired length will not allow at least (1) space between words');
    }
                     // 18 / 9 = 2
    let spaceSize = numSpaces / words.length; // The number of spaces allocated per word.
                //"The"
    let buffer = words.shift(); //Take the first word off the array and push it into the buffer.
                  // "dog"
    let lastWord = words[words.length - 1];

    for(let word of words)  { //Loop through the set of words
        if(word !== lastWord)
        {
            for(let i = 0;i< spaceSize;i++){
                buffer += " ";

            }
            buffer += word; //Push the non-last word into the buffer.
        }else{
            console.debug('buffer length at last word =>', buffer.length);
            // This is really in-eloquent, but it's the best I can think of on short notice
            // Buffer the last *before* the last word.
            let finalSpaceSize =  len - buffer.length - word.length;
            //For the last word we pre-pend the spaces instead of appending them.
            for(let i = 0;i< finalSpaceSize;i++){
                buffer += " ";
            }
            buffer += word; 
        }
        

    }
    console.debug(buffer);
    return buffer;

}

module.exports = justify;

package.json

{
  "name": "coding_challenge",
  "version": "1.0.0",
  "description": "Coding Challenge",
  "main": "index.js",
  "scripts": {
    "test": "jest ./test.js"
  },
  "author": "Michael Draper",
  "license": "ISC",
  "devDependencies": {
    "jest": "^22.4.0"
  }
}

test.js

//Author: Michael Draper <mdraper@gmail.com>
// Tests the justify() function
// These tests were coded for Jest (eg. npm install --save-dev jest)

const justify = require('./index');


test('accepts a buffer and prints out exactly that buffer length', function(){
    let output = justify('The quick brown fox jumps over the lazy dog', 52);


    expect(output[0]).toEqual('T');
    expect(output[51]).toEqual('g');
    expect(output.length).toEqual(52);
});

test('works for a phrase different than the sample case from the exercise', function(){
  let output = justify('King of the world!', 22);
  expect(output[0]).toEqual('K');
  expect(output[21]).toEqual('!');
  expect(output.length).toEqual(22);
})

test('throws an error when we cannot have enough spaces', function(){
  const error = function(){
    justify('The quick brown fox jumps over the lazy dog', 41);
  } 

  expect(error).toThrowError(/the specified length is too short to justify/);

});

test('does not allow non-string as input', function(){
  const error = function(){
    justify({'heh': 'this should not work'}, 42);
  } 

  expect(error).toThrowError(/input is not a string/);

});

test('does not allow non-integer for second parameter', function(){
  const error = function(){
    justify("The Last Jedi", '42');
  } 

  expect(error).toThrowError(/len is not a number/);

});

test('does not allow negative numbers', function(){
  const error = function(){
    justify("The Last Jedi", -42);
  } 

  expect(error).toThrowError(/the specified length is too short to justify/);

});

test('does not allow cheeky bastards to pass floating point numbers', function(){
  const error = function(){
    justify("The Last Jedi", 3.1419);
  } 

  expect(error).toThrowError(/len must be a whole number/);

});