I'm writing an AWS Lambda function in TypeScript using the Node.js runtime. I'm using a "batchDelete" function from a DynamoDB ORM library which returns an AsyncIterableIterator type.
According to the documentation here https://github.com/awslabs/dynamodb-data-mapper-js#batchDelete, I should invoke the method with a for await loop like this:
for await (const found of mapper.batchDelete(toRemove)) {
// items will be yielded as they are successfully removed
}
This all works great but the problem comes in where if I enable ESLint on my project. The default rules throw an error because the for await block is empty. I also get a warning because the found constant is never used. I have no use for the found constant and don't want to log it. I was wondering if there was another way to call an AsyncIterableIterator function where we disregard what is returned and don't have the empty block?
If you don't care about the results of the iteration, then you should probably just do something like this:
await Promise.all(toRemove.map(item => mapper.delete(item));
To use the mapper.batchDelete(toRemove) result more directly, you have to allow for multiple levels of promises. Perhaps you could do this:
await Promise.all(await mapper.batchDelete(toRemove)[Symbol.asyncIterator]());
In doing this, await mapper.batchDelete(toRemove)[Symbol.asyncIterator](), that would get you the default async Iterator and then passing it to Promise.all() would iterate it to get an iterable of promises. Unfortunately, in building it to make this easier:
for await (const found of mapper.batchDelete(toRemove))
they made it a bit more difficult to just get an array of promises out of it.
FYI, here's a link to the code for the .batchDelete() method if you want to look at how it's implemented.
Related
I am a bit new to JavaScript web dev, and so am still getting my head around the flow of asynchronous functions, which can be a bit unexpected to the uninitiated. In my particular use case, I want execute a routine on the list of available databases before moving into the main code. Specifically, in order to ensure that a test environment is always properly initialized, I am dropping a database if it already exists, and then building it from configuration files.
The basic flow I have looks like this:
let dbAdmin = client.db("admin").admin();
dbAdmin.listDatabases(function(err, dbs){/*Loop through DBs and drop relevant one if present.*/});
return await buildRelevantDB();
By peppering some console.log() items throughout, I have determined that the listDatabases() call basically puts the callback into a queue of sorts. I actually enter buildRelevantDB() before entering the callback passed to listDatabases. In this particular example, it seems to work anyway, I think because the call that reads the configuration file is also asynchronous and so puts items into the same queue but later, but I find this to be brittle and sloppy. There must be some way to ensure that the listDatabases portion resolves before moving forward.
The closest solution I found is here, but I still don't know how to get the callback I pass to listDatabases to be like a then as in that solution.
Mixing callbacks and promises is a bit more advanced technique, so if you are new to javascript try to avoid it. In fact, try to avoid it even if you already learned everything and became a js ninja.
Dcumentation for listDatabases says it is async, so you can just await it without messing up with callbacks:
const dbs = await dbAdmin.listDatabases();
/*Loop through DBs and drop relevant one if present.*/
The next thing, there is no need to await before return. If you can await within a function, it is async and returns a promise anyway, so just return the promise from buildRelevantDB:
return buildRelevantDB();
Finally, you can drop database directly. No need to iterate over all databases to pick one you want to drop:
await client.db(<db name to drop>).dropDatabase();
Top-level await support was added to Node.js in 14.3.0 via --experimental-top-level-await and later to --harmony-top-level-await.
The Problem
I need to use a top level await in my ESM script file, if it is supported by the current Node.js runtime. And further, I need to set a boolean flag to indicate that the promise was successfully awaited at the top level.
An example of what I mean:
let topLevelAwaitEnabled;
try {
await Promise.resolve(); // replaced with an actual promise
topLevelAwaitEnabled = true;
} catch (ignored) {
topLevelAwaitEnabled = false;
}
console.log(topLevelAwaitEnabled);
// carry on with the rest of the application, regardless of success or failure
// at some point, topLevelAwaitEnabled is checked to conditionally execute some code
If top level await support is enabled, this succeeds fine. However, if it is not supported, this will result in the following error during parsing and cannot be caught at runtime with a try/catch:
$ node test.js...\test.js:3
await Promise.resolve(); // replaced with an actual promise
^^^^^
SyntaxError: await is only valid in async function
So the question is: How can I use a top level await if it is supported, without creating incompatibility issues with Node.js runtimes that do not support top level await (either no CLI flag was specified or simply no runtime support)?
If the answer is "it is not possible", I would like an explanation as to why this is impossible.
In the case I am actually committing an XY problem, the underlying issue is I need a top-level dynamic import.
Note: I am well aware that top level await is not recommended for a variety of reasons, however it is crucial for a specific functionality of my application and does not impose any issue with my use case. Alternatives will likely not suffice.
Attempts
I have tried the following methods, to no avail:
eval: I have tried replacing the await line with an eval("await Promise.resolve()"), in the hope the code was evaluated in the current context. Unfortunately, even if top level await is supported, this will result in the same error, as it does not seem to inherit the current context.
vm.compileFunction: Same issue was eval(), top level await is not supported.
vm.SourceTextModule: Evaluation is asynchronous and would need to be awaited at the top level to check if it is supported... which is a catch 22.
conditional execution of the await based on process.version and process.execArgv: The error during parsing - it never actually executes the code, so conditional execution is ruled out.
As far as I know this is not possible because the parser will simply error out. The compiler will not understand the await directive and will not complete its cycle. This is probably similar to using a word that's simply not a recognized keyword.
The closest you can get is using an anonymous function.
Seems like you might be able to check the version of node being used at runtime like so process.version and then you can use also use process.argv to check for any flags passed in when starting the process.
Using the above methods, you can first check the version of node being used, and then if relevant, you can check for the needed flags.
Node docs on checking for CLI args.
(async()=>{await promise})()
I am still quite new to Node.js and can't seem to find anything to help me around this.
I am having an issue of getting the query from my last record and adding it to my variable.
If I do it like below: -
let lastRecord = Application.find().sort({$natural:-1}).limit(1).then((result) => { result });
Then I get the value of the variable showing in console.log as : -
Promise { <pending> }
What would I need to do to output this correctly to my full data?
Here is it fixed:
Application.findOne().sort({$natural:-1}).exec().then((lastRecord) => {
console.log(lastRecord); // "lastRecord" is the result. You must use it here.
}, (err) => {
console.log(err); // This only runs if there was an error. "err" contains the data about the error.
});
Several things:
You are only getting one record, not many records, so you just use findOne instead of find. As a result you also don't need limit(1) anymore.
You need to call .exec() to actually run the query.
The result is returned to you inside the callback function, it must be used here.
exec() returns a Promise. A promise in JavaScript is basically just a container that holds a task that will be completed at some point in the future. It has the method then, which allows you to bind functions for it to call when it is complete.
Any time you go out to another server to get some data using JavaScript, the code does not stop and wait for the data. It actually continues executing onward without waiting. This is called "asynchronisity". Then it comes back to run the functions given by then when the data comes back.
Asynchronous is simply a word used to describe a function that will BEGIN executing when you call it, but the code will continue running onward without waiting for it to complete. This is why we need to provide some kind of function for it to come back and execute later when the data is back. This is called a "callback function".
This is a lot to explain from here, but please go do some research on JavaScript Promises and asynchronisity and this will make a lot more sense.
Edit:
If this is inside a function you can do this:
async function someFunc() {
let lastRecord = await Application.findOne().sort({$natural:-1}).exec();
}
Note the word async before the function. This must me there in order for await to work. However this method is a bit tricky to understand if you don't understand promises already. I'd recommend you start with my first suggestion and work your way up to the async/await syntax once you fully understand promises.
Instead of using .then(), you'll want to await the record. For example:
let lastRecord = await Application.find().sort({$natural:-1}).limit(1);
You can learn more about awaiting promises in the MDN entry for await, but the basics are that to use a response from a promise, you either use await or you put your logic into the .then statement.
I am trying to understand the node.js documentation specifically for the https.get() method. https://nodejs.org/dist/latest-v8.x/docs/api/https.html#https_https_get_options_callback
What is unclear to me is the callback. The example in the document indicates the callback can take a res (response) object as its parameter but I am unsure if this is the only parameter it can take or more importantly where I can find the definition of the res object so I can know what properties and methods I can access on this object.
Is there a straightforward way to identify this?
I have read this thread: Trying to understand nodejs documentation. How to discover callback parameters and the answers seem to suggest that if there is a non-error argument that a callback can take it will be documented, but I am assuming that answer is outdated.
I've run into the same issue with many Node/NPM packages. Documentation sometimes does not describe the parameters well.
So, welcome to JavaScript in 2018! It's gotten a lot better, though, to be honest.
My go-to method is to try the methods and dump the information myself.
Try a console.dir(res) in your callback:
https.get('https://encrypted.google.com/', (res) => {
console.dir(res);
});
Alternatively, you can set a breakpoint in the callback and inspect it yourself. You can then probe the arguments object* to see what else, if anything, was passed as an argument, or do another console dump:
https.get('https://encrypted.google.com/', function (res) {
console.dir("args:", arguments);
console.dir("res:", res);
});
EDIT: Wait, apparently the arguments variable is not available to arrow functions, fixed the second example.
*From MDN:
The arguments object is not an Array. It is similar to an Array, but
does not have any Array properties except length.
From your link https://nodejs.org/dist/latest-v8.x/docs/api/https.html#https_https_get_options_callback, you can see that it works like the http version :
Like http.get() but for HTTPS.
With http.get() clickable.
On that page (https://nodejs.org/dist/latest-v8.x/docs/api/http.html#http_http_get_options_callback), we can see this :
The callback is invoked with a single argument that is an instance of http.IncomingMessage
With http.IncomingMessage clickable, linking this page :
https://nodejs.org/dist/latest-v8.x/docs/api/http.html#http_class_http_incomingmessage
I agree the Node documentation is not very clear about the callbacks in general, and that is a shame. You can still use IDEs with good intellisense (and JSDoc to identify the type of the function parameters), like VSCode.
Or you can use a debugger, always works :)
Edit: If you want to see all the parameters sent to a function, you can use the spread syntax like this :
function foo(...params) {
// Here params is an array containing all the parameters that were sent to the function
}
If you want the absolute truth, you can look at the implementation. Though that's fairly time consuming.
If you find that the documentation is wrong, or in this case could be improved by adding a sentence about the callback parameter to https.get(), please open an issue, or, better yet, a pull request. This is where the change needs to be made:
https://github.com/nodejs/node/blob/67790962daccb5ff19c977119d7231cbe175c206/doc/api/https.md
I needed to compare two arrays the first one a couple of filenames from a database, the second one a list of files I already downloaded to my client. The Idea was to load whatever files are missing on the client.
As the reading via fswas two slow, I tried using Promises to wait for one function to finish before the next starts. But somehow I got lost...
My code so far:
let filesIneed = [];
let filesIhave = [];
let filesToFetch = [];
getLocalFiles().then(getFilesIneed).then(getfilesToRetreive);
function getLocalFiles() {
fs.readdir(localPath, (err, files) => {
files.forEach(file => {
filesIhave.push(file)
});
})
return Promise.all(filesIhave);
}
function getFilesIneed () {
for (let x of docs) {//this is my JSON
filesIneed.push(y.NameOfFileIShouldHave);
}
}
return Promise.all(filesIneed);
}
function getfilesToRetreive() {
filesToFetch = _.difference(filesIneed, filesIhave);
return Promise.all(filesToFetch);
}
console.log(filesToFetch);
I do get the first and second array ("filesIneed" and "filesIhave"), but difference is always empty. So maybe I just mangled up the Promises, as this concept is completely new to me and I'm aware I only understood half of it.
This is completely wrong. You cannot run Promise.all on an array of filenames. You can only run it on an array of promises.
There is also no need to push every element of an array one at a time to an empty array just to return that array when you already have that array in the first place.
You cannot use promises to compare two arrays. You can use lodash to compare two arrays in a then handler of a promise, that resolves to an array.
If you want to get a promise of file names from the fs.readdir then use one of the following modules:
https://www.npmjs.com/package/mz
http://bluebirdjs.com/docs/api/promise.promisifyall.html
https://www.npmjs.com/package/fs-promise
https://www.npmjs.com/package/fs-promised
Also don't use global variables for everything because you will have problems with any concurrency.
Also, read about promises. Without understanding how promises work you will not be able to guess a correct way of using them. Even looking at some working code examples can help a lot and there are a lot of questions and answers on stack Overflow about promises:
promise call separate from promise-resolution
Q Promise delay
Return Promise result instead of Promise
Exporting module from promise result
What is wrong with promise resolving?
Return value in function from a promise block
How can i return status inside the promise?
Should I refrain from handling Promise rejection asynchronously?
Is the deferred/promise concept in JavaScript a new one or is it a traditional part of functional programming?
How can I chain these functions together with promises?
Promise.all in JavaScript: How to get resolve value for all promises?
Why Promise.all is undefined
function will return null from javascript post/get
Use cancel() inside a then-chain created by promisifyAll
Why is it possible to pass in a non-function parameter to Promise.then() without causing an error?
Implement promises pattern
Promises and performance
Trouble scraping two URLs with promises
http.request not returning data even after specifying return on the 'end' event
async.each not iterating when using promises
jQuery jqXHR - cancel chained calls, trigger error chain
Correct way of handling promisses and server response
Return a value from a function call before completing all operations within the function itself?
Resolving a setTimeout inside API endpoint
Async wait for a function
JavaScript function that returns AJAX call data
try/catch blocks with async/await
jQuery Deferred not calling the resolve/done callbacks in order
Returning data from ajax results in strange object
javascript - Why is there a spec for sync and async modules?
Return data after ajax call success