javascript - Promise 中异步执行的代码在哪里？

Question

以下 JavaScript 函数来自 You Don't Know JS: Async & Performance 。根据我的理解，第一条评论 //start do things that might take a while 具有误导性。实际可能异步完成某些操作的代码部分位于传递到 Promise 的函数中。构造函数。

function foo(x) {
    // start doing something that could take a while *misleading comment*

    // construct and return a promise
    return new Promise( /* executor */ function(resolve,reject){
        // eventually, call `resolve(..)` or `reject(..)`,
        // which are the resolution callbacks for
        // the promise.
    } );
}

我会通过以下方式修复它:

function foo(x) {
    // construct and return a promise
    return new Promise( /* executor */ function(resolve,reject){
        // start doing something that could take a while
        // then foo returns the newly created Promise
        // eventually, call `resolve(..)` or `reject(..)`,
        // which are the resolution callbacks for
        // the promise.
    } );
}

Answer 1

是的，这个问题应该得到解决(感谢您提交 this issue )。

这让我想起了一点 difference between the deferred pattern and the revealing constructor pattern 。在 Promise 构造函数回调中启动异步任务有两个优点:

• 如果它同步抛出(例如语法错误、方法调用中的拼写错误等)，异常将被隐式捕获并拒绝 promise
• resolve 和 reject 回调已在范围内，可以作为回调传递给异步进程。