当关键字后面的表达式没有评估为承诺时，`await`会发生什么？ [英] What should happen with `await` when the expression after the keyword does not evaluate to promise?

问题描述

我有这样的ES7代码。

I have a ES7 code like this.

async function returnsfive() {
  var three = 3;
  var threeP = await three;
  return threeP+2;
}

returnsfive().then(k=>console.log(k), e=>console.error("err", e))

在 var threeP =等待三个行会发生什么？

如果代码按预期继续，或者失败，因为三不是承诺？

Should the code continue as expected, or fail, because three is not a promise?

在此回购中，它被称为Debatable Syntax& amp; ;语义学。我无法阅读官方文档以找到确切的定义，因为它太技术性了。

In this repo, it is mentioned as "Debatable Syntax & Semantics". I am not able to read through the official documentation to find the exact definition, since it's too technical.

默认babel.js转换日志5按预期进行;但是， nodent - 一个不同的转换 - 打印 TypeError：three.then不是函数。这是正确的，为什么？

Default babel.js transformation logs 5 as expected; however, nodent - a different transform - prints TypeError: three.then is not a function. Which is correct and why?

推荐答案

根据当前工作草案规范，运行时应首先将等待值转换为承诺：

According to the current working draft spec, the runtime should "cast" the awaited value to a promise first:

AsyncFunctionAwait（ value ）

AsyncFunctionAwait ( value )

1. 让 asyncContext 成为正在运行的执行上下文。


2. 让 promiseCapability 为！ NewPromiseCapability（％Promise％）。


3. 让 resolveResult 成为！调用（promiseCapability。[[Resolve]]，undefined，value）。


4. ...

1. Let asyncContext be the running execution context.
2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
3. Let resolveResult be ! Call(promiseCapability.[[Resolve]], undefined, value).
4. ...

步骤2和3的组合大致相当于调用 Promise.resolve（value） ，它创建一个使用给定值解析的新promise或 - 如果值是一个很好的 - 将跟随那个。

Step 2 and 3 combined is roughly equivalent to calling Promise.resolve(value), which creates a new promise that is resolved with the given value or - if the value is a thenable - will follow that thenable.

换句话说： await 3 相当于等待Promise.resolve（3），并且Babel正确实现了规范。

In other words: await 3 is equivalent to await Promise.resolve(3), and Babel implements the spec correctly.

nodent另一方面故意不支持默认等待非承诺。如果您希望所有等待的值首先包含在promise中，则可以使用 wrapAwait 选项，但nodent文档报告这可能会影响性能。

nodent on the other hand deliberately does not support awaiting a non-promise by default. There's a wrapAwait option available if you want all awaited values to be wrapped in a promise first, but the nodent documentation reports that this may affect performance.

这篇关于当关键字后面的表达式没有评估为承诺时，`await`会发生什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！