Why can I await this code but not use .then?

Question

Node.JS 10.15, serverless, lambdas, invoked locally

SAMPLE A) This Works:

export async function main(event) {
    const marketName = marketIdToNameMap[event.marketId];
    const marketObject = marketDirectory[marketName];
    const marketClient = await marketObject.fetchClient();

    const marketTime = await marketObject.getTime(marketClient);
    console.log(marketTime);
}

SAMPLE B) and this works:

export function main(event) {
    const marketName = marketIdToNameMap[event.marketId];
    const marketObject = marketDirectory[marketName];

    marketObject.fetchClient().then((marketClient)=>{
        marketObject.getTime(marketClient).then((result) => {
            console.log('<---------------> marker 1 <--------------->');
            console.log(result);
        });
    });
}

SAMPLE C) but this does not:

export async function main(event) {
    const marketName = marketIdToNameMap[event.marketId];
    const marketObject = marketDirectory[marketName];
    const marketClient = await marketObject.fetchClient();

    console.log('<---------------> marker 1 <--------------->');

    marketObject.getTime(marketClient).then((result) => {
        console.log('<---------------> marker 22 <--------------->');
        console.log(result);
    });
}

the guts of getTime are for all examples:

function getTime(marketClient){
    return new Promise((resolve, reject) => {
        return marketClient.getTime((err, result) => {
            if (err) {
                reject(err);
            }
            resolve(result);
        });
    }).catch(err => {
        throw err;
    });
}

clearly, it seems to be an issue with mixing async/awaits with classic promise then-ables. I would expect SAMPLE C to work because getTime() is returning a promise. However, the code simply finishes silently, never hitting the second marker. I have to put the first marker there just to be sure any code is run at all. It feels like i should be able to mix async/await and thenables, but i must not be considering something here.

@adrian, nope

Answer 1

You're neither awaiting nor returning the promise from marketObject.getTime().then(), and this will result in that promise chain executing independently, the main function returning and the process closing. Remember.. then returns a promise too.

the solution is

await marketObject.getTime(marketClient).then(...

or

return marketObject.getTime(marketClient).then(...

either way chains the promise to the main function such that whatever executes it consistently waits for all promises to resolve (or reject).

I suspect Sample B works because the main is not async and Lambda will wait for the event-loop to complete. i.e. it will execute the promise chain even though main returned early.

https://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-handler.html

If you don't use callback in your code, AWS Lambda will call it implicitly and the return value is null. When the callback is called, AWS Lambda continues the Lambda function invocation until the event loop is empty.

... and I suspect if you return a Promise (as you do in Sample C) then Lambda will simply terminate the process immediately once it resolves, which is does because you don't await/return the .then() chain, and thus the floating promise chain you've created will not execute.