setTimeout 在类中声明时只执行两次？ [复制]

Question

为什么这只执行两次？

class TestClass {
    runPeriodically() {
        console.log('hello!');
        setTimeout(this.runPeriodically, 1000);
    }
}

let test = new TestClass();
test.runPeriodically();

Answer 1

class TestClass {
    runPeriodically() {
        console.log('hello!');
        setTimeout(()=> this.runPeriodically(), 1000);
    }
}

let test = new TestClass();
test.runPeriodically();