title: Why are there undefineds in my output?
body: <p>Consider the following code:</p>
  
  <pre><code>function displayParkReport() {
    let title, part1, part2, part3;
  
    title = '---PARKS REPORT---\n';
    part1 = `Our ${data.all.parks.length} parks have an average of ${calcAverageParkAge()} years.`;
  
    data.all.parks.forEach(function(cur) {
      part2 += `${cur.name} has a tree density of ${cur.calcParkDensity()} trees per square km. \n`;
    });
  
    data.all.parks.forEach(function(cur) {
      if(cur.numberOfTrees &gt; 1000) {
        part3 += `${cur.name} has more than 1000 trees. \n`;
      }
    });
  
    console.log(title + part1 + part2 + part3);
  }
  
  function displayStreetReport() {
    let title, part1, part2;
  
    title = '---STREETS REPORT---\n';
    part1 = `Our ${data.all.streets.length} streets have a total length of ${calcTotalStreetLength()} km, with an average of ${calcAverageStreetLength()} km. \n`;
  
    data.all.streets.forEach(function(cur) {
      part2 += `${cur.name}, built in ${cur.builtYear}, is a ${cur.size} street.\n`;
    });
  
    console.log(title + part1 + part2);
  }
  </code></pre>
  
  <p>Here is the output:</p>
  
  <pre><code>---PARKS REPORT---
  Our 5 parks have an average of 43.600 years.undefinedNorth Park has a 
  tree density of 62.500 trees per square km. 
  Central Park has a tree density of 102.174 trees per square km. 
  West Park has a tree density of 70.833 trees per square km. 
  East Park has a tree density of 100.000 trees per square km. 
  Hasan Efendi has a tree density of 235.714 trees per square km. 
  undefinedCentral Park has more than 1000 trees. 
  East Park has more than 1000 trees. 
  Hasan Efendi has more than 1000 trees. 
  
  ---STREETS REPORT---
  Our 4 streets have a total length of 122 km, with an average of 30.5 km. 
  undefinedBroadway, built in 2012, is a huge street.
  Cavanough, built in 1992, is a normal street.
  CrazyJoe, built in 2001, is a tiny street.
  WestPark, built in 1956, is a small street.
  </code></pre>
  
  <p>Why is there random <code>undefined</code>s in the output?</p>