class Range {

    constructor(start, end) {

        this.start = start;

        this.end = end;

    }

    *[Symbol.iterator]() {

        for(let i = this.start; i < this.end; i++) {

            yield i;

        }

    }

    toString() {

        return `Range(${this.start}...${this.end - 1})`;

    }   

}

let range = new Range(1, 4);   // 建構式 start = 1, end = 4

for(let i of range) {

    console.log(i);            // 顯示 1 2 3

}

console.log(range.toString()); // 顯示 Range(1...3)