高度なJavaScript for文のテクニック：効率的なコードを書くためのノウハウ

for (初期化式; 条件式; 更新式) {
// 繰り返し実行するコード
}

for (let i = 1; i <= 5; i++) {
console.log(i);
}

1
2
3
4
5

const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = [];

for (let i = 0; i < numbers.length; i++) {
doubledNumbers.push(numbers[i] * 2);
}

console.log(doubledNumbers);

[2, 4, 6, 8, 10]

const person = {
name: 'John',
age: 30,
occupation: 'Engineer'
};

for (let key in person) {
console.log(`${key}: ${person[key]}`);
}

name: John
age: 30
occupation: Engineer

for (let i = 1; i <= 5; i++) {
if (i % 2 === 0) {
continue; // 偶数の場合、次のループへスキップ
}
console.log(i);
}

1
3
5

// for...of文の例
const fruits = ['apple', 'orange', 'banana'];

for (const fruit of fruits) {
console.log(fruit);
}

// for...in文の例
const person = {
name: 'Jane',
age: 25,
occupation: 'Designer'
};

for (const key in person) {
console.log(`${key}: ${person[key]}`);
}

apple
orange
banana

name: Jane
age: 25
occupation: Designer

const scores = [85, 90, 78, 92, 88];
let sum = 0;

for (let i = 0; i < scores.length; i++) {
sum += scores[i];
}

const average = sum / scores.length;
console.log(`合計: ${sum}, 平均: ${average}`);

合計: 433, 平均: 86.6

for (let i = 1; i <= 9; i++) {
for (let j = 1; j <= 9; j++) {
const result = i * j;
console.log(`${i} × ${j} = ${result}`);
}
}

1 × 1 = 1
1 × 2 = 2
...
9 × 8 = 72
9 × 9 = 81