等待一个非承诺有任何可察觉的影响吗？

一个人可以 await一个非承诺和那就好。

所有这些表达式都是有效的，不会导致错误:

await 5
await 'A'
await {}
await null
await undefined

是否有任何 可探测的效应等待非承诺？为了避免潜在的错误，在行为上有什么不同吗？表现有什么不同吗？

下面两行是完全相同还是 理论上是的不同? :

var x = 5
var x = await 5

怎么做? 有什么例子能说明区别吗？

PS: 根据 TypeScript 作者，两者是有区别的:

var x = await 5;与 var x = 5;不同; var x = await 5;将在下一个结构中赋值 x5，其中 var x = 5;将立即求值。

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最佳答案

await is not a no-op. If the awaited thing is not a promise, it is wrapped in a promise, that promise is awaited. Therefore await changes the execution order (but you should not rely on it nevertheless):

console.log(1);
(async function() {
var x = await 5; // remove await to see 1,3,2
console.log(3);
})();
console.log(2);

Additionally await does not only work on instanceof Promises but on every object with a .then method:

await { then(cb) { /* nowhere */ } };
console.log("will never happen");

Is there any detectable effect of awaiting a non-Promise?

Sure, .then gets called if it exists on the awaited thing.

Is there any difference in behavior one should be aware of to avoid a potential error?

Don't name a method "then" if you don't want it to be a Promise.

Any performance differences?

Sure, if you await things you will always defer the continuation to a microtask. But as always: You won't probably notice it (as a human observing the outcome).