標準メソッド 解答例

let str = "Hello, world!";
let substring = str.substring(0, 5);
console.log(substring); // "Hello"

let arr = [1, 2, 3, 4, 5];
let slicedArr = arr.slice(1, 4);
console.log(slicedArr); // [2, 3, 4]

let str = "apple";
let newStr = str.concat("pie");
console.log(newStr); // "applepie"

let arr = [1, 2, 3, 4, 5];
let doubledArr = arr.map((num) => num * 2);
console.log(doubledArr); // [2, 4, 6, 8, 10]

let str = "Hello, world!";
let count = (str.match(/o/g) || []).length;
console.log(count); // 2

let str = "Hello, world!";
let reversedStr = str.split("").reverse().join("");
console.log(reversedStr); // "!dlrow ,olleH"

let arr = [1, 2, 3, 4, 5];
let sum = arr.reduce((total, num) => total + num, 0);
console.log(sum); // 15

let str = "apple";
let index = str.indexOf("p");
console.log(index); // 1