Since NodeJS v12 async stack traces are enabled by default.

- [relevant issue](https://github.com/nodejs/node/issues/11865)

- [how async stacktraces work](https://docs.google.com/document/d/13Sy_kBIJGP0XT34V1CV3nkWya4TwYx9L3Yv45LdGB6Q/edit)

My website has a player for multiple videos.  I&#39;ve been adapting the code to use YouTube&#39;s iframe API as the player.  I can&#39;t get the videos to autoplay.  Here&#39;s the relevant code:

        &lt;body&gt;
              &lt;iframe id=&quot;existing-iframe-example&quot;
                  width=&quot;640&quot; height=&quot;360&quot;
                  src=&quot;https://www.youtube.com/embed/-SFcIUEvNOQ?   autoplay=1&amp;;enablejsapi=1&quot;
                  frameborder=&quot;0&quot;
                  autoplay=&quot;1&quot;
                  style=&quot;border: solid 4px #37474F&quot;
                  &gt;&lt;/iframe&gt;
            &lt;div id=&quot;player&quot;&gt;&lt;/div&gt;
    
            &lt;div id=&quot;movieButtons&quot; class=&quot;movieButtons&quot;&gt;
			&lt;button class=&quot;movieButton&quot; data-movieAdr=&quot;1-VjtC939_Q&quot;&gt;Memorial Slide Show&lt;/button&gt;

The rest is standard YouTube iframe API script.  You can see I tried setting Autoplay=1 as both an iframe parameter and as part of the iframe src.  I&#39;ve also tried including it in the data in the button.  None of these seems to work.  The vids load when the buttons are clicked, but they don&#39;t autoplay.




YouTube Autoplay not working

I am setting up my own Django server using this [Digital Ocean tutorial][1]. I created the Django framework following each step, and ran the server using this command:

    ./manage.py runserver 0.0.0.0:8000

When I tried to visit the IP at port 8000, the following error was shown:

    DisallowedHost at /
    Invalid HTTP_HOST header: &#39;XXX.XXX.XXX.XXX:8000&#39;. You may need to add u&#39;XXX.XXX.XXX.XXX&#39; to ALLOWED_HOSTS.
(IP substituted with X&#39;s)

Why is this happening?
 
  [1]: https://www.digitalocean.com/community/tutorials/how-to-set-up-django-with-postgres-nginx-and-gunicorn-on-ubuntu-14-04

Why is Django throwing error &quot;DisallowedHost at /&quot;?

We&#39;re experimenting with using `--harmony_async_await` in Node 7, and compared to transpiling with babel for async/await are missing the ability to have long stack traces (http://bluebirdjs.com/docs/api/promise.longstacktraces.html).

Obviously, it would be great if there was an option to &#39;just see this&#39; - on the chrome(ium) side they appear to support it in the inspector, though the latest work on this is after v8.5.4 (i.e. not in Node 7):

https://bugs.chromium.org/p/v8/issues/detail?id=4483

https://codereview.chromium.org/2357423002/

An alternative approach would be to get v8 to use bluebird&#39;s promise implementation - but how could I do that? Overriding `global. Promise` doesn&#39;t change how async/await behave (they still use the default promises).

Is it possible to get stack traces across async/await boundaries using --harmony_async_await in Node 7?

We're experimenting with using <code>--harmony_async_await</code> in Node 7, and compared to transpiling with babel for async/await are missing the ability to have long stack traces (<a href="http://bluebirdjs.com/docs/api/promise.longstacktraces.html" target="_blank" rel="noopener noreferrer">http://bluebirdjs.com/docs/api/promise.longstacktraces.html</a>).
Obviously, it would be great if there was an option to 'just see this' - on the chrome(ium) side they appear to support it in the inspector, though the latest work on this is after v8.5.4 (i.e. not in Node 7):
<a href="https://bugs.chromium.org/p/v8/issues/detail?id=4483" target="_blank" rel="noopener noreferrer">https://bugs.chromium.org/p/v8/issues/detail?id=4483</a>
<a href="https://codereview.chromium.org/2357423002/" target="_blank" rel="noopener noreferrer">https://codereview.chromium.org/2357423002/</a>
An alternative approach would be to get v8 to use bluebird's promise implementation - but how could I do that? Overriding <code>global. Promise</code> doesn't change how async/await behave (they still use the default promises).

Came to know that from **React v15.3.0**, we have a new base class called **PureComponent** to extend with **PureRenderMixin** built-in. What I understand is that, under the hood this employs a shallow comparison of props inside `shouldComponentUpdate`.

Now we have 3 ways to define a React component:

 1. Functional stateless component which doesn&#39;t extend any class
 2. A component that extends `PureComponent` class
 3. A normal component that extends `Component` class

Some time back we used to call stateless components as Pure Components, or even Dumb Components. Seems like the whole definition of the word &quot;pure&quot; has now changed in React.

Although I understand basic differences between these three, I am still not sure **when to choose what**. Also what are the performance impacts and trade-offs of each?


----------


### **Update**:
These are the question I expect to get clarified:

 - Should I choose to define my simple components as functional (for the sake of simplicity) or extend `PureComponent` class (for performance sake)?
 - Is the performance boost that I get a real trade-off for the
   simplicity I lost?
 - Would I ever need to extend the normal `Component` class when I can always use `PureComponent` for better performance?

React functional stateless component, PureComponent, Component; what are the differences and when should we use what?

According to [this artcle](http://paqmind.com/posts/rxjs-error-and-completed-events-demystified/), `onComplete` and `onError` function of the `subscribe` are mutually exclusive.  

Meaning either `onError` or `onComplete` events will fire up in my `subscribe`.  
I have a logic block which needs to be executed whether I receive an error, or I finish my steam of information successfully.  

I looked up for something like [`finally` in python](https://docs.python.org/3/tutorial/errors.html), but all I found is [`finally`](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/finally.md) which needs to be attached to the observable I create.   

But I want to to do that logic only when I subscribe, and after the stream has ended, whether successfully or with an error.

Any ideas?

Observable Finally on Subscribe

I&#39;m using VueJS to make a simple enough resource management game/interface. At the minute I&#39;m looking to activate the `roll` function every 12.5 seconds and use the result in another function. 
At the moment though I keep getting the following error:

&gt; Uncaught TypeError: Cannot read property &#39;roll&#39; of undefined(...)

I have tried:

- `app.methods.roll(6);`
- `app.methods.roll.roll(6);`
- `roll.roll()`
- `roll()`

but can&#39;t seem to access the function. Anyone any ideas how I might achieve this?

    methods: {

      // Push responses to inbox.
      say: function say(responseText) {
        console.log(responseText);
        var pushText = responseText;
        this.inbox.push({ text: pushText });
      },

      // Roll for events
      roll: function roll(upper) {
        var randomNumber = Math.floor(Math.random() * 6 * upper) + 1;
        console.log(randomNumber);
        return randomNumber;
      },

      // Initiates passage of time and rolls counters every 5 time units.
      count: function count() {
        function counting() {
          app.town.date += 1;
          app.gameState.roll += 0.2;

          if (app.gameState.roll === 1) {
            var result = app.methods.roll(6);
            app.gameState.roll === 0;
            return result;
          }
        }

        setInterval(counting, 2500);

        ...

        // Activates the roll at times.
      }
    }

VueJS accessing a method from another method

I have a function that helps filter data. I am using `v-on:change` when a user changes the selection but I also need the function to be called even before the user selects the data. I have done the same with `AngularJS` previously using `ng-init` but I understand that there is no such a directive in `vue.js` 

This is my function:

    getUnits: function () {
            var input = {block: this.block, floor: this.floor, unit_type: this.unit_type, status: this.status};

            this.$http.post(&#39;/admin/units&#39;, input).then(function (response) {
                console.log(response.data);
                this.units = response.data;
            }, function (response) {
                console.log(response)
            });
        }

In the `blade` file I use blade forms to perform the filters:

    &lt;div class=&quot;large-2 columns&quot;&gt;
            {!! Form::select(&#39;floor&#39;, $floors,null, [&#39;class&#39;=&gt;&#39;form-control&#39;, &#39;placeholder&#39;=&gt;&#39;All Floors&#39;, &#39;v-model&#39;=&gt;&#39;floor&#39;, &#39;v-on:change&#39;=&gt;&#39;getUnits()&#39; ]) !!}
        &lt;/div&gt;
        &lt;div class=&quot;large-3 columns&quot;&gt;
            {!! Form::select(&#39;unit_type&#39;, $unit_types,null, [&#39;class&#39;=&gt;&#39;form-control&#39;, &#39;placeholder&#39;=&gt;&#39;All Unit Types&#39;, &#39;v-model&#39;=&gt;&#39;unit_type&#39;, &#39;v-on:change&#39;=&gt;&#39;getUnits()&#39; ]) !!}
        &lt;/div&gt;

This works fine when I select a specific item. Then if I click on all lets say `all floors`, it works. What I need is when the page is loaded, it calls the `getUnits` method which will perform the `$http.post` with empty input. In the backend I have handled the request in a way that if the input is empty it will give all the data. 

How can I do this in `vuejs2`?

My Code: http://jsfiddle.net/q83bnLrx 



How to call a vue.js function on page load

I have a form that has a submit button.
That form calls a function onclick that sets the state of something from false to true.
I then want to pass this state back to the parent so that if it is true it renders componentA but if it is false it renders componentB.

How would I do that in react?
I know I need to use state or props but not sure how to do it. also is this contradicting the one-way flow react principle??

ComponentA code:

    &lt;form onSubmit={this.handleClick}&gt;
  

    handleClick(event) {
        this.setState({ decisionPage: true });
        event.preventDefault();
      };

Parent component that controls what it displays:

    return (
          &lt;div&gt;
          {this.props.decisionPage ?
            &lt;div&gt;
              &lt;LoginPage /&gt;
            &lt;/div&gt;
            :
            &lt;div&gt;
              &lt;Decision showThanks={this.props.showThanks}/&gt;
            &lt;/div&gt;
          }
          &lt;/div&gt;
        )



How to pass state back to parent in React?

I&#39;m new to Docker and I&#39;m having a hard time to setup the docker container as I want. I have a nodejs app can take two parameters when start. For example, I can use

 `node server.js 0 dev` 

or

 `node server.js 1 prod`

 to switch between production mode and dev mode and determine if it should turn the cluster on. Now I want to create docker image with arguments to do the similar thing,  the only thing I can do so far is to adjust the Dockerfile to have a line 

`CMD [ &quot;node&quot;, &quot;server.js&quot;, &quot;0&quot;, &quot;dev&quot;]` 

and 

`docker build -t me/app .` to build the docker.

Then `docker run -p 9000:9000 -d me/app` to run the docker.

But If I want to switch to prod mode, I need to change the Dockerfile CMD to be 

`CMD [ &quot;node&quot;, &quot;server.js&quot;, &quot;1&quot;, &quot;prod&quot;]` , 

and I need to kill the old one listening on port 9000 and rebuild the image.
I wish I can have something like 

`docker run -p 9000:9000 environment=dev cluster=0 -d me/app` 

to create an image and run the nodejs command with &quot;environment&quot; and &quot;cluster&quot; arguments, so I don&#39;t need to change the Dockerfile and rebuild the docker any more. How can I accomplish this?

Use docker run command to pass arguments to CMD in Dockerfile

I&#39;m digging into the node 7 async/await feature and keep stumbling across code like this

    

&lt;!-- begin snippet: js hide: false console: true babel: false --&gt;

&lt;!-- language: lang-js --&gt;

    function getQuote() {
      let quote = &quot;Lorem ipsum dolor sit amet, consectetur adipiscing elit laborum.&quot;;
      return quote;
    }

    async function main() {
      try {
        var quote = await getQuote();
        console.log(quote);
      } catch (error) {
        console.error(error);
      }
    }

    main();

&lt;!-- end snippet --&gt;

This seems to be the only possibility resolve/reject or return/throw with async/await, however, v8 doesn&#39;t optimise code within try/catch blocks?!

Are there alternatives?

try/catch blocks with async/await

I want to use mocha to test my TypeScript/Angular2 project.  I tried to use **ts-node** as described [here](https://stackoverflow.com/a/35661569/5115768):  

    npm install -g ts-node

but when running  

    mocha --require ts-node/register -t 10000 ./**/*.unit.ts

I get an error  
&gt; Cannot find module &#39;ts-node/register&#39;
  
What am I missing here?

Cannot find module &#39;ts-node/register&#39;

Is it possible in **ES6** to set a variable inside of a `try{}` using `const` in strict mode?

    &#39;use strict&#39;;

    const path = require(&#39;path&#39;);

    try {
		const configPath = path.resolve(process.cwd(), config);
    } catch(error) {
		//.....
    }

    console.log(configPath);

This fails to lint because `configPath` is defined out of scope. The only way this seems to work is by doing:

    &#39;use strict&#39;;

    const path = require(&#39;path&#39;);

    let configPath;
    try {
		configPath = path.resolve(process.cwd(), config);
    } catch(error) {
		//.....   
    }

    console.log(configPath);

Basically, is there any way to use `const` instead of `let` for this case?

Javascript set const variable inside of a try block

I am using Mocha for Unit tests.

When testing begins, I would like to delete all the previous records in a table.  

What I have tried:

    db.User.destroy({ force: true }).then(() =&gt; {
    }).then(() =&gt; done());


    db.User.destroy(
        {where: undefined},
        {truncate: false}
    ).then(() =&gt; {
        return 
    }).then(() =&gt; done());


    db.User.destroy({}).then(() =&gt; {
        return db.User.bulkCreate(users)
    }).then(() =&gt; done());


I keep getting the following error:

     Error: Missing where or truncate attribute in the options parameter of model.destroy.


How do I delete/destroy all the records in a table?


Sequelize: Destroy/Delete all records in the table

I have few blocking functions `foo`, `bar` and I can&#39;t change those (Some internal library I don&#39;t control. Talks to one or more network services). How do I use it as async?. E.g. I wan&#39;t to do the following.

    results = []
    for inp in inps:
        val = foo(inp)
        result = bar(val)
        results.append(result)

This will be inefficient as I can call `foo` for the second input while I am waiting for the first and same for `bar`. How do I wrap them such that they are usable with asyncio (i.e new `async`, `await` syntax)?

Lets assume the functions are re-entrant. i.e it is fine to call `foo` again when already a previous `foo` is processing.


----------

**Update**

Extending answer with reusable decorator. Click [here](https://stackoverflow.com/a/53719009/) for example.

    def run_in_executor(f):
        @functools.wraps(f)
        def inner(*args, **kwargs):
            loop = asyncio.get_running_loop()
            return loop.run_in_executor(None, functools.partial(f, *args, **kwargs))
    
        return inner

How to use asyncio with existing blocking library?

Seem to be having some issues incorporating async/await with .reduce(), like so:

    const data = await bodies.reduce(async(accum, current, index) =&gt; {
      const methodName = methods[index]
      const method = this[methodName]
      if (methodName == &#39;foo&#39;) {
        current.cover = await this.store(current.cover, id)
        console.log(current)
        return {
          ...accum,
          ...current
        }
      }
      return {
        ...accum,
        ...method(current.data)
      }
    }, {})
    console.log(data)

The `data` object is logged *before* the `this.store` completes...

I know you can utilise `Promise.all` with async loops, but does that apply to `.reduce()`?

JavaScript array .reduce with async/await

I&#39;m using *Express.js* in my code with *Node.js* v7.3. In this I&#39;ve created a `User Router` which forwards the requests to my `User Controller`.

I&#39;m using async/await inside the `User Controller` to do asynchronous calls. The problem is that IntelliJ gives me a warning saying that 

&gt; Promise returned from login() is ignored.

The thing is I&#39;m not even returning anything from the `login()` method.

Here&#39;s the code - 

**UserRouter.js**

    router.post(&#39;/login&#39;, function (req, res, next) {
        userController.login(req, res); // I get the warning here
    });

**UserController.js**

    exports.login = async function (req, res) {
        try {
            const verifiedUser = await someFunction(req.body.access_code);
            let user = await User.findOrCreateUser(verifiedUser);
            res.status(200).send(user);
        }
        catch (err) {
            res.status(400).send({success: false, error: err});
        }
    };


If I write the same login method using native promises only then I don&#39;t get this warning. Am I understanding something wrong here or is IntelliJ at fault?

**EDIT -**

Thanks to @Stephen, I understand that an async function returns a promise but wouldn&#39;t it be better if Intellij identifies that nothing is being returned from the async function and doesn&#39;t show that warning because when I chain a `.then()` after the `login()` function, it provides an `undefined` object into the then result. It means if we don&#39;t return something from the async function explicitly then undefined is returned?

Intellij Idea warning - &quot;Promise returned is ignored&quot; with aysnc/await

I&#39;m using `async`/`await` to fire several `api` calls in parallel:

    async function foo(arr) {
      const results = await Promise.all(arr.map(v =&gt; {
         return doAsyncThing(v)
      }))
      return results
    }

I know that, unlike `loops`, `Promise.all` [executes in-parallel][1] (that is, the waiting-for-results portion is in parallel). 

But [I also know that][2]:

&gt; Promise.all is rejected if one of the elements is rejected and
&gt; Promise.all fails fast: If you have four promises which resolve after
&gt; a timeout, and one rejects immediately, then Promise.all rejects
&gt; immediately.


As I read this, if I `Promise.all` with 5 promises, and the first one to finish returns a `reject()`, then the other 4 are effectively cancelled and their promised `resolve()` values are lost.

Is there a third way? Where execution is effectively in-parallel, but a single failure doesn&#39;t spoil the whole bunch? 



 
 


  [1]: https://stackoverflow.com/questions/35612428/call-async-await-functions-in-parallel
  [2]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all

How do I await multiple promises in-parallel without &#39;fail-fast&#39; behavior?

I am trying to log a long array so I can copy it quickly in my terminal. However, if I try and log the array it looks like:

    [&#39;item&#39;,
     &#39;item&#39;,
      &gt;&gt;more items&lt;&lt;&lt;
      ... 399 more items ]

How can I log the entire array so I can copy it really quickly? 

Dumping whole array: console.log and console.dir output &quot;... NUM more items]&quot;

I&#39;ve just discovered the ECMAScript 7 feature `a**b` as an alternative for `Math.pow(a,b)` ([MDN Reference][1]) and came across a discussion in [that post][2], in which they apparently behave differently. I&#39;ve tested it in Chrome 55 and can confirm that the results differ.

`Math.pow(99,99)` returns `3.697296376497263e+197`

whereas

`99**99` returns `3.697296376497268e+197`

So logging the difference `Math.pow(99,99) - 99**99` results in `-5.311379928167671e+182`.

So far it could be said, that it&#39;s simply another implementation, but wrapping it in a function behaves different again:

    function diff(x) {
      return Math.pow(x,x) - x**x;
    }

calling `diff(99)` returns `0`.

Why is that happening?

As [xszaboj][3] pointed out, this can be narrowed down to this problem:

    var x = 99;
    x**x - 99**99; // Returns -5.311379928167671e+182

  [1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Exponentiation_(**)
  [2]: http://www.2ality.com/2016/02/exponentiation-operator.html#comment-2946471782
  [3]: https://stackoverflow.com/users/3454771/xszaboj


Why is Math.pow() (sometimes) not equal to ** in JavaScript?

I&#39;ve been thinking about this question for a while and can&#39;t seem to find the answer. What is the relationship between Node.js and V8? and Can Node.js work without V8?

What is the relationship between Node.js and V8?

I am asking this question because I and my colleague have a dispute on coding style because he prefers arrows function declaration:

    const sum = (a, b) =&gt; a + b;

And I prefer old-style standalone function declaration:

    function sum(a, b) {
        return a + b;
    }

My point is that code in old-style more readable and you can more clearly distinguish function and variable declarations. His point is that code with arrow functions just run faster.

Do you know something about actual performance penalties (in v8) when you use old-style standalone function declaration instead of arrow functions? Are that penalties really exists?

Are arrow functions faster (more performant, lighter) than ordinary standalone function declaration in v8?

I&#39;m having some trouble to understand how Node.js acts based on the parameter `max-old-space-size`.

In my case, for example, I&#39;m running two `t2.small` AWS instances (2GB of RAM).

Not sure why, but I did set `max-old-space-size=4096` (4GB). What does node do in this case? Could this configuration lead to a possible memory allocation failure?

How do I determine the correct value of `max-old-space-size` based on the server resources?

My application is constantly growing the memory usage and I&#39;m trying to understand everything about node internals.

How do I determine the correct &quot;max-old-space-size&quot; for node.js?

I have trouble understanding the difference between putting `.catch` BEFORE and AFTER then in a nested promise.

Alternative 1:

    test1Async(10).then((res) =&gt; {
      return test2Async(22)
        .then((res) =&gt; {
          return test3Async(100);
        }).catch((err) =&gt; {
          throw &quot;ERROR AFTER THEN&quot;;
        });
    }).then((res) =&gt; {
      console.log(res);
    }).catch((err) =&gt; {
      console.log(err);
    });

Alternative 2:

    test1Async(10).then((res) =&gt; {
       return test2Async(22)
         .catch((err) =&gt; {
            throw &quot;ERROR BEFORE THEN&quot;;
          })
          .then((res) =&gt; {
            return test3Async(100);
          });
      }).then((res) =&gt; {
        console.log(res);
      }).catch((err) =&gt; {
        console.log(err);
      });

The behavior of each function is as follow, test1 fail if number is `&lt;0` test2 fails if number is `&gt; 10` and test3 fails if number is not `100`. In this case test2 is only failing.

I tried to run and make test2Async fail, both BEFORE and AFTER then behaves the same way and that is not executing the test3Async. Can somebody explain to me the  main difference for placing catch in different places? 

In each function I `console.log(&#39;Running test X&#39;)` in order to check if it gets executed. 

This question arises because of the previous thread I posted https://stackoverflow.com/questions/41982433/how-to-turn-nested-callback-into-promise/41984015?noredirect=1#comment71158471_41982637. I figure it is a different problem and worth posting another topic.

Content Type	Original Author	Original Content on Stackoverflow
Question	james.haggerty	View Question on Stackoverflow
Solution 1 - Javascript	Jonas Wilms	View Answer on Stackoverflow

Is it possible to get stack traces across async/await boundaries using --harmony_async_await in Node 7?

Javascript Problem Overview

Javascript Solutions

Solution 1 - Javascript

Why is Django throwing error "DisallowedHost at /"?

YouTube Autoplay not working

Attributions