Async functions targeting ES5

Question

I've noticed something strange while using TypeScript 1.8. I'm targeting ES5 and I was trying to use async/await. I know support for that is on the roadmap for 2.0, but it seems to partially work already?

For example, this doesn't compile:

module MyModule { async function myFunction() { ... } }

The error is error TS1311: Async functions are only available when targeting ECMAScript 6 and higher. Pretty clear, right?

However, this does compile, and seems to work fine:

module MyModule { myFunction = async function () { ... } }

This doesn't make sense to me. Why would the latter syntax work fine, while the former explictly refuses to compile? Is there a subtle difference to these two syntaxes that I'm missing?

Sounds more like a shortcoming in TypeScript's error reporting. Both snippets output JavaScript files with ES6 generators, so neither of them actually works on ES5. — Mattias Buelens
– Mattias Buelens, Commented Jun 5, 2016 at 18:07
@MattiasBuelens If an engine were to strictly limit itself to ES5, it wouldn't work no. Many modern browsers as well as Node.js have at least partial ES6 support though. In my case it was quite useful, as targeting ES6 would mean that other (unsupported) things would be used too. — Thorarin
– Thorarin, Commented Jun 5, 2016 at 22:54

basarat · Accepted Answer · 2016-06-06 01:41:24Z

4

They are not support as es5 targets yet. Will happen soon though.

More

I mention this fact here : https://basarat.gitbooks.io/typescript/content/docs/async-await.html

answered Jun 6, 2016 at 1:41

basarat

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1 Comment

alek kowalczyk Over a year ago

How soon is soon? :)

btpka3 · Accepted Answer · 2017-01-02 13:47:47Z

Found a comment on 《What about Async/Await?》by Oskar, I tried the way he mentioned :

TypeScript => ES6 => Babel => ES5.

Using [email protected] (using [email protected]), and [email protected], the result is : chrome, firefox, Safari, IE Edge works fine, except IE8.

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