All Articles

神奇的Node.js map中+=async问题

1. 看代码

东西都不复杂，看代码就完事了，一共是4个代码片段

map 中进行 += await

const f = (x) => Promise.resolve(x);

(async () => {
  let total = 0;
  await Promise.all(
    [1, 1, 1].map(async (x, i) => {
      console.log('exec', i);
      console.log('b', total);
      total += await f(x);
      console.log('a', total);
    })
  );
  console.log('done', total);
})();

结果如图:

map 中进行 =赋值 然后+=

const f = (x) => Promise.resolve(x);

(async () => {
  let total = 0;
  await Promise.all(
    [1, 1, 1].map(async (x, i) => {
      console.log('exec', i);
      console.log('b', total);
      const r = await f(x);
      total += r;
      console.log('a', total);
    })
  );
  console.log('done', total);
})();

结果如图:

map 中进行 = await +

const f = (x) => Promise.resolve(x);

(async () => {
  let total = 0;
  await Promise.all(
    [1, 1, 1].map(async (x, i) => {
      console.log('exec', i);
      console.log('b', total);
      total = await f(x) + total;
      console.log('a', total);
    })
  );
  console.log('done', total);
})();

这个结果和图2相同，也就是和上面一组相同

map 中进行同步 +=

const f = (x) => x;

(async () => {
  let total = 0;
  await Promise.all(
    [1, 1, 1].map(async (x, i) => {
      console.log('exec', i);
      console.log('b', total);
      total += f(x);
      console.log('a', total);
    })
  );
  console.log('done', total);
})();

结果如图:

2. 原理

原理一点都不复杂，稍微想一下就知道了。这里主要是要注意这种case的发生，需要有意识地规避。因为结果比较反直觉，就算是有经验的程序员也会犯错。

EOF

Published 2022/9/5

Some tech & personal blog posts