# event-loop与promise

```javascript
/**
 * 请写出两种或两种以上实现方法满足: execute 对应的id按顺序打印
 * PS: 尝试只修改start函数体
 *
 * 输出结果参考:
 * id 0
 * id 1
 * id 2
 * id 3
 * id 4
 */

function start(id) {
  execute(id).catch(console.error);
}

// 测试代码 (请勿更改):

for (let i = 0; i < 5; i++) {
  start(i);
}

function sleep() {
  const duration = Math.floor(Math.random() * 500);
  return new Promise(resolve => setTimeout(resolve, duration));
}

function execute(id) {
  return sleep().then(() => {
    console.log("id", id);
  });
}
```

{% tabs %}
{% tab title="First Tab" %}

{% endtab %}

{% tab title="答案" %}

```javascript
function start(id) {
  start.promises = !start.promises
    ? execute(id)
    : start.promises.then(() => execute(id));
}
```

{% endtab %}

{% tab title="答案2" %}

```javascript
Array.from({ length: 5 }).reduce(
  (chain, _, id) => chain.then(() => execute(id)),
  Promise.resolve()
)
```

{% endtab %}
{% endtabs %}


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