array.split to create a string with multiple whitespaces

Question

I am trying to create several strings comprising of varying amounts of whitespaces only. I have been able to do this once with array.(number of indexes).join(" ");

However when I go into a loop to add more strings with an incrementing amount of white space in each string, the string is set to 'undefined.' I am not sure where I am going wrong. The function seems to work outside of the loop but not inside. Any ideas what I have done wrong?

function towerBuilder(nFloors) {
    // build here
    var towers = [];
    var stars = "*";
    var spaceNo = nFloors -1;
    let spaces = Array(spaceNo).join(" ");
    spaceNo -= 1;
    towers[0] = spaces + stars + spaces;
    for(i = 1; i <= nFloors -1; i++)
    {
      stars = stars + Array(i + 2).join('*');
      let edges = Array(spaceNo).join(" ");
      towers[i] = edges + stars + edges;
      spaceNo -= 1;
    }
    

    return towers;
}
console.log(towerBuilder(3));

At the top, this works:

 let spaces = Array(spaceNo).join(" ");

However, down in the loop, this outputs 'undefined'

let edges = Array(spaceNo).join(" ");

The idea is that the spaces wrap the star symbols so that a tower can be built out of an array. Example, calling towerBuilder(3) would output the below: (I don't need to output it with carriage returns. The array alone will suffice.)

[
  '  *  ', 
  ' *** ', 
  '*****'
]

"owever, down in the loop, this outputs 'undefined'" Not in the code you've posted. I took it and dropped it into a Stack Snippet, and it doesn't do what you describe. Please update your question with a minimal reproducible example demonstrating the problem (ideally using Stack Snippets as I did; here's more info about them). — T.J. Crowder
– T.J. Crowder, Commented Dec 4, 2017 at 9:04
What output are you getting? I am expecting to see spaces wrapped around the stars on each line but I only get the below. [ " * ", "***", "******" ] — Nick Proud
– Nick Proud, Commented Dec 4, 2017 at 9:32
Which isn't undefined. Your question says "this outputs 'undefined'". It doesn't. — T.J. Crowder
– T.J. Crowder, Commented Dec 4, 2017 at 9:34

Mario Lamacchia · Accepted Answer · 2017-12-04 09:23:37Z

Array.join is used to concatenate all the array's values, with the given string between them:

['foo','bar'].join(',') // = 'foo,bar'

You are expecting Array(2).join(' ') to be two spaces, but it will be only one since it's concatenating two empty values with a space in between.

Just remove the -1 at row 5 and it will work as expected:

function towerBuilder(nFloors) {
    // build here
    var towers = [];
    var stars = "*";
    var spaceNo = nFloors; // was nFloors - 1
    let spaces = Array(spaceNo).join(" ");
    spaceNo -= 1;
    towers[0] = spaces + stars + spaces;
    for(i = 1; i <= nFloors -1; i++)
    {
      stars = stars + Array(i + 2).join('*');
      let edges = Array(spaceNo).join(" ");
      towers[i] = edges + stars + edges;
      spaceNo -= 1;
    }
    

    return towers;
}
console.log(towerBuilder(3));

Btw, it would look a lot better if written like this:

function towerBuilder(nFloors) {
    // build here
    var towers = [];
    
    for(var floor = 0; floor < nFloors; floor++)
    {
      var nStars = (floor * 2) + 1;
      var stars = Array(nStars + 1).join('*');
      var edges = Array(nFloors - floor).join(" ");
      towers[floor] = edges + stars + edges;
    }

    return towers;
}
console.log(towerBuilder(3));

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