出力に未定義があるのはなぜですか？

Question

は、次のコードを考えてみましょう。ここで

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 > 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); 
} 

が出力されます。

---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. 

なぜ出力でランダムundefined sがありますか？

Answer 1

a += bを使用しているのは、a = a + bと同じです。最初にアクセスしたときにpart2とpart3が定義されていないため、part2 = undefined + '...'を実行しようとしています。undefinedにつながります。それらに初期値を割り当てます：

let part2 = ''; 
let part3 = ''; 

最初にアクセスしたときにそれらの値が定義されていないので、空の文字列になります。