记Promise一个容易被忽略的特性

在看webpack bootstrap代码的时候，发现new Promise传的函数是会立即执行的

function test() {
    let arr = [1, 2, 3, 4];
    let tasks = arr.map(item => {
        console.log(`task ${item}`)
        let p = new Promise(async function(resolve) {
            console.log(`pre ${item}`)
            await delay(item);  // line 2
            console.log(`outside ${item}`); // line 3
            resolve(item);
        });
        console.log(`after promise ${item}`)
        return p
    });

    Promise.all(tasks).then(() => console.log("done")); // line 4
}

打印结果：

1. 立即出现的部分:

   task 1
   pre 1
   after promise 1
   task 2
   pre 2
   after promise 2
   task 3
   pre 3
   after promise 3
   task 4
   pre 4
   after promise 4
   

2. 延后5秒出现的部分：

   outside 1
   outside 2
   outside 3
   outside 4
   

发现是自己2了，此文灌水了~