Downlevel Async Functions

This feature was supported before TypeScript 2.1, but only when targeting ES6/ES2015.TypeScript 2.1 brings the capability to ES3 and ES5 run-times, meaning you’ll be free to take advantage of it no matter what environment you’re using.

Note: first, we need to make sure our run-time has an ECMAScript-compliant Promise available globally.That might involve grabbing a polyfill for Promise, or relying on one that you might have in the run-time that you’re targeting.We also need to make sure that TypeScript knows Promise exists by setting your lib flag to something like "dom", "es2015" or "dom", "es2015.promise", "es5"

Example

tsconfig.json

{
    "compilerOptions": {
        "lib": ["dom", "es2015.promise", "es5"]
    }
}

dramaticWelcome.ts

function delay(milliseconds: number) {
    return new Promise<void>(resolve => {
        setTimeout(resolve, milliseconds);
    });
}
async function dramaticWelcome() {
    console.log("Hello");
    for (let i = 0; i < 3; i++) {
        await delay(500);
        console.log(".");
    }
    console.log("World!");
}
dramaticWelcome();

Compiling and running the output should result in the correct behavior on an ES3/ES5 engine.