using await on global scope without async keyword

问题:

I am trying to do something like this on global scope in nodejs REPL. As per my understanding both the following statements are valid. see docs

let x = await Promise.resolve(2);
let y = await 2;

However, both these statements are throwing an error.

Can somebody explain why? my node version is v8.9.4

回答1:

await can only be used within a function that is labeled async, so there are two ways you can approach this.

Note: There is a proposal in place that may eventually allow the usage of Top level await calls.

The first way is to create a self invoked function like this:

(async function() {
  let x = await Promise.resolve(2)
  let y = await 2
  
  console.log(x, y)
})()

Or the second way is to use .then()

Promise.resolve(2).then(async data => {
  let x = data
  let y = await 2

  console.log(x, y)
})

回答2:

You can not do that. MDN doc says

The await operator is used to wait for a Promise. It can only be used inside an async function.

回答3:

This proposal is currently in stage 2 of the TC39 process. LINK

You can use this feature in Google Chrome and Mozilla Firefox as of now. You can use top level await without async in console.

https://twitter.com/addyosmani/status/1080365576218759168

回答4:

since node 10, you can run node process with --experimental-repl-await to allow to level await https://nodejs.org/api/repl.html#repl_await_keyword

回答5:

You could wrap all the code in the global scope in an async function.

For example:

// ...global imports...

new Promise (async () => {

  // ...all code in the global scope...

}).then()