5.4 Async & Await

>>>

async function foo() {
    return 'foo';
}

The async function or async () => (arrow function) syntax modifies the semantics for a function.

>>>

foo()

Promise(status="fulfilled", result="foo")

Behind the scenes, an async function always returns a Promise.

>>>

await foo()

"foo"

Using await val the execution stops until the promise has been resolved or rejected.

>>>

async function bar() {
    throw new Error('bar');
}

Inside an async function, return and throw are automatically converted to a corresponding Promise.resolve() or Promise.reject() statement.

>>>

async function baz() {
    try {
        await foo();
        await bar();
        return '\u{1f389}';
    } catch (e) {
        return '\u{1f44e} ' + e;
    }
}

The resulting code therefore looks eerily similar to any synchronous code.

>>>

await baz()

"👎 Error: bar"

As long as await is not omitted, everything works as expected. And we'll never have to create a Promise again!

>>>

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