I had the error `expression expected` reported by *Webstorm* when editing a Node.js program. In this case the solution is to set the language version to a version that supports this feature.

[![enter image description here][1]][1]


  [1]: http://i.stack.imgur.com/XKuiS.png

The following is what i did that work for me.
(1) I change the JavaScript language option to ECMAScript 6 as show in the selected answer by @Joe23

(2) I close the Webstorm project/application.

(3) Navigate to the project folder and delete the **.idea** folder in it. I believe this is the folder webstorm generated to keep information about the project/application.

(4) I reopen my project in webstorm and the errors are gone.



Arrow functions are available by default on the latest versions of node and other javascript runtimes. You need to enable support for them only if you&#39;re dealing with a really old runtime (0.12 and earlier if I recall correctly) in which case you need to add the &quot;--harmony&quot; flag when you start the node process.

With regard to these great two sources: [NZakas - Returning Promises in Promise Chains](https://github.com/nzakas/understandinges6/blob/master/manuscript/11-Promises.md#returning-promises-in-promise-chains) and [MDN Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise), I would like to ask the following:

Each time that we return a value from a promise fulfillment handler, how is that value passed to the new promise returned from that same handler?

For instance,

    let p1 = new Promise(function(resolve, reject) {
        resolve(42);
    });
    
    let p2 = new Promise(function(resolve, reject) {
        resolve(43);
    });
    
    let p3 = p1.then(function(value) {
        // first fulfillment handler
        console.log(value);     // 42
        return p2;
    });
    
    p3.then(function(value) {
        // second fulfillment handler
        console.log(value);     // 43
    });

In this example, `p2` is a promise. `p3` is also a promise originating from `p1`&#39;s fulfillment handler. However `p2 !== p3`. Instead `p2` somehow magically resolves to `43` (how?) and that value is then passed to `p3`&#39;s fulfillment handler. Even the sentence here is confusing.

Could you please explain to me what exactly is going on here? I am totally confused over this concept.

Promise callbacks returning promises

I have installed Rails and Ruby on Windows with railsinstaller. The problem is, when I run the rails command, it gives me: &quot;The system cannot find the path specified.&quot;

I am running windows 7 x64 and Ruby 2.20.

I tried uninstalling Rails and installing it again; that does not help. Ruby commands execute, like `ruby -v`, but `rails -v=` dont work. 


Rails - The system cannot find the path specified

I want to transform this code:

    var formatQuoteAmount = function (tx) {
        return Currency.toSmallestSubunit(tx.usd, &#39;USD&#39;);
    };
    var quoteAmounts = res.transactions.map(formatQuoteAmount);


into an anonymous arrow function. I&#39;ve written this:
    
    var quoteAmounts = res.transactions.map(tx =&gt; Currency.toSmallestSubunit(tx.usd, &#39;USD&#39;));

I get `expression expected` syntax error at the arrow. I looked up [the default syntax here][1] and seems like the syntax of my code is correct. Any ideas what the problem might be?

I have it working with this syntax:
    
        var quoteAmounts = res.transactions.map(function (tx) {
            return Currency.toSmallestSubunit(tx.usd, &#39;USD&#39;)
        });

but I want to make it a one-liner, with an arrow-function.

Running on `node v5.3.0`

  [1]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions#Shorter_functions

Arrow function &quot;expression expected&quot; syntax error

I want to transform this code:
<pre><code class="hljs language-ini">var formatQuoteAmount = function (tx) {
 return Currency.toSmallestSubunit(tx.usd, 'USD');
};
var quoteAmounts = res.transactions.map(formatQuoteAmount);
</code></pre>
into an anonymous arrow function. I've written this:
<pre><code class="hljs language-ini">var quoteAmounts = res.transactions.map(tx => Currency.toSmallestSubunit(tx.usd, 'USD'));
</code></pre>
I get <code>expression expected</code> syntax error at the arrow. I looked up <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions#Shorter_functions" target="_blank" rel="noopener noreferrer">the default syntax here</a> and seems like the syntax of my code is correct. Any ideas what the problem might be?
I have it working with this syntax:
<pre><code class="hljs language-javascript"> var quoteAmounts = res.transactions.map(function (tx) {
 return Currency.toSmallestSubunit(tx.usd, 'USD')
 });
</code></pre>
but I want to make it a one-liner, with an arrow-function.
Running on <code>node v5.3.0</code>

I&#39;ve tried searching the internet for the execution order of imported modules. For instance, let&#39;s say I have the following code:

    import &quot;one&quot;
    import &quot;two&quot;
    console.log(&quot;three&quot;);

Where `one.js` and `two.js` are defined as follows:

    // one.js
    console.log(&quot;one&quot;);
    
    // two.js
    console.log(&quot;two&quot;);

Is the console output guaranteed to be:

    one
    two
    three

Or is it undefined?

What is the defined execution order of ES6 imports?

##tl;dr; About the Same Origin Policy

I have a Grunt process which initiates an instance of express.js server. This was working absolutely fine up until just now when it started serving a blank page with the following appearing in the error log in the developer&#39;s console in Chrome (latest version):
  
&gt;XMLHttpRequest cannot load https://www.example.com/
    No &#39;Access-Control-Allow-Origin&#39; header is present on the requested 
    resource. Origin &#39;http://localhost:4300&#39; is therefore not allowed access.

What is stopping me from accessing the page?

XMLHttpRequest cannot load XXX No &#39;Access-Control-Allow-Origin&#39; header

How would one use moment.js to get the number of days in the current month? Preferably without temporary variables.

Using moment.js to get number of days in current month

Can anyone help me to change Gregorian date to Persian in JavaScript? I want to use it in HTML and JavaScript.

How to change Gregorian date to Persian date in JavaScript?

I was toying a bit and was trying to instantiate a new array of length `x`, where all elements of that array were initialized to a value `y`:

    var arr = new Array(x).fill(y);

This works well if the value of `y` is anything other than **an object**.
Meaning that if `y` is an object, the following is true:

    var arr = new Array(2).fill({});
    arr[0] === arr[1]; //is true;
    arr[0].test = &#39;string&#39;;
    arr[1].test === &#39;string&#39;; //is also true;

Is there any way to state that a new object should be created for each element while using the fill-function? Or should I just convert it to a loop?

Array.prototype.fill() with object passes reference and not new instance

As far as I understand, in ES7/ES2016 putting multiple `await`&#39;s in code will work similar to chaining `.then()` with promises, meaning that they will execute one after the other rather than in parallel. So, for example, we have this code:

    await someCall();
    await anotherCall();

Do I understand it correctly that `anotherCall()` will be called only when `someCall()` is completed? What is the most elegant way of calling them in parallel?

I want to use it in Node, so maybe there&#39;s a solution with async library?

EDIT: I&#39;m not satisfied with the solution provided in this question: https://stackoverflow.com/questions/24193595/slowdown-due-to-non-parallel-awaiting-of-promises-in-async-generators, because it uses generators and I&#39;m asking about a more general use case.

Call async/await functions in parallel

I am using MongoDB with node js , i used `npm install mongodb`

I want to update an existing document and return the updated document , the document is updated correctly . but it returns the old document means the original document before update . i have used the `returnNewDocument:true` parameter but no use .

                var filter = {
                    &#39;_id&#39;: object_id
                },
                update = {
                    $set: { &quot;status&quot; : data[&quot;status&quot;] },
                    $push: {
                        &quot;statusHistory&quot;: {
                            $each: [{ status:data[&quot;status&quot;],statusChangedTime:data[&quot;statusChangedTime&quot;],comment:data[&quot;comment&quot;]}],
                            $position:0,
                        }
                    },
                }
                ,options = {
                    //upsert: false,
                    //multi: false,
                    returnNewDocument: true
                };

                col.findOneAndUpdate(filter, update, options,function(err, res) {
                    if (err) {

                        console.log(err);
                    }else {

                        console.log(res);
                    }
                });





the response is 




    { lastErrorObject: { updatedExisting: true, n: 1 },
      value: 
       { 
    //original document
       },     
      ok: 1 }



when i directly go to mongoDB through terminal and try

    db.MyCollection.find().pretty();

the document is updated correctly, it just returns the original instead of updated one. 

Stuck here for 2 hours, any help is appreciated  

in package.json 

    &quot;mongodb&quot;: &quot;^2.1.4&quot;,

How to get updated document back from the findOneAndUpdate method?

I am using TypeScript with Express/Node.js.

For consuming modules, the TypeScript Handbook shows the following syntax:

```
import express = require(&#39;express&#39;);
```

But also the `typescript.d.ts` file shows:

```
import * as express from &quot;express&quot;;
```

I also searched the MSDN blog but could not find anything.

Which one is more correct as of early 2016? What are the differences between the two, if any?

Where is the best source to find information on the latest syntax to use so I can find this information in the future?

TypeScript import/as vs import/require?

I&#39;m on Windows 10, with Node 5.6.0 and npm 3.6.0. I&#39;m trying to install angular-material and mdi into my working folder. **npm install angular-material mdi** errors with:

    +-- angular@1.5.0

    +-- UNMET PEER DEPENDENCY angular-animate@^1.5.0

    +-- UNMET PEER DEPENDENCY angular-aria@^1.5.0

    +-- angular-material@1.0.6

    +-- UNMET PEER DEPENDENCY angular-messages@^1.5.0 `-- mdi@1.4.57

    npm WARN enoent ENOENT: no such file or directory, open
    &#39;C:\Users\xxxxx\Desktop\ngClassifieds\package.json&#39; 

    npm WARN angular-material@1.0.6 requires a peer of
    angular-animate@^1.5.0 but none was installed. 

    npm WARN angular-material@1.0.6 requires a peer of angular-aria@^1.5.0
    but none was installed. 

    npm WARN angular-material@1.0.6 requires a peer of
    angular-messages@^1.5.0 but none was installed.

How do I resolve this to get AngularJS Material and MDI installed?

How do I fix the npm UNMET PEER DEPENDENCY warning?

As of ES6, JavaScript has a proper Map object. I don&#39;t see a way to use a literal notation though, as you could with an Array or an Object. Am I missing it, or does it not exist?

Array: `var arr = [&quot;Foo&quot;, &quot;Bar&quot;];`

Object: `var obj = { foo: &quot;Foo&quot;, bar: &quot;Bar&quot; };`

Map: ???

Does JavaScript have a Map literal notation?

I&#39;d like to the following but with a single line, if possible:

- `import Module from &#39;./Module/Module;&#39;`
- `export Module;`

I tried the following but it doesn&#39;t seem to work:

- `export Module from &#39;./Module/Module;`

ES6, how can you export an imported module in a single line?

I have the following code and I get the error &#39;Duplicate Declaration query_url&#39;.

      switch(condition) {
        case &#39;complex&#39;:
          const query_url = `something`;
          break;
        default:
          const query_url = `something`;
          break;
      }

I understand that query_url is getting declared twice which isn&#39;t right. But i don&#39;t know how to resolve this. Can someone please help on what should be the correct way to make this work?

Error Duplicate Const Declaration in Switch Case Statement

The export statement below gives a syntax error 

    export default const hello = () =&gt; console.log(&quot;say hello&quot;)

why ?

I&#39;m only able to export named functions

    export function hello() {
      console.log(&quot;hello&quot;)
    }

What is the reason?



Is it possible to export Arrow functions in ES6/7?

I&#39;m running lint with my React app, and I receive this error:

    error    JSX props should not use arrow functions        react/jsx-no-bind

And this is where I&#39;m running the arrow function (inside `onClick`):

    {this.state.photos.map(tile =&gt; (
      &lt;span key={tile.img}&gt;
        &lt;Checkbox
          defaultChecked={tile.checked}
          onCheck={() =&gt; this.selectPicture(tile)}
          style={{position: &#39;absolute&#39;, zIndex: 99, padding: 5, backgroundColor: &#39;rgba(255, 255, 255, 0.72)&#39;}}
        /&gt;
        &lt;GridTile
          title={tile.title}
          subtitle={&lt;span&gt;by &lt;b&gt;{tile.author}&lt;/b&gt;&lt;/span&gt;}
          actionIcon={&lt;IconButton onClick={() =&gt; this.handleDelete(tile)}&gt;&lt;Delete color=&quot;white&quot;/&gt;&lt;/IconButton&gt;}
        &gt;
          &lt;img onClick={() =&gt; this.handleOpen(tile.img)} src={tile.img} style={{cursor: &#39;pointer&#39;}}/&gt;
        &lt;/GridTile&gt;
      &lt;/span&gt;
    ))}

Is this a bad practice that should be avoided? And what&#39;s the best way to do it?

Why shouldn&#39;t JSX props use arrow functions or bind?

I recently switched to es6 and started using arrow functions all over my code.
While refactoring I came across below code

    data.map(function(d) {
       return {id: d.id, selected: bool};
    });

I changed above code to this -

    data.map((d) =&gt; {id: d.id, selected: bool});

But I was getting error from above code. I don&#39;t know what is wrong here?
I know that If there is no block of code then there is implicit return provided by arrow function.

But don&#39;t know how to return empty object or anonymous object with some properties initialized ?

**Edit:**

What is wrong if I do it in this way? Just for curiosity sake.

    data.map((d) =&gt; new {id: d.id, selected: bool});



How to return anonymous object from one liner arrow function in javascript?

So we can do:

    export function myMethod (param: number) :number
    export function myMethod (param: string) :string
    
    export function myMethod (param: string | number): string | number {
      if (typeof param === &#39;string&#39;) {
        return param.toUpperCase()
      } else {
        return param + 1
      }
    }



Can I declare and implement it with arrow function?

    export var myMethodArror = (param: string): string
    export var myMethodArror = (param: number): number
    
    export var myMethodArror = (param: string | number): string | number =&gt; {
    ..
    }

I am aware of that it is not possible to duplicate the variables declaration, but my question is: is it possible to make function overload using arrow notation?



Typescript overload arrow functions

I&#39;m new to both ES6 and React and I keep seeing arrow functions. Why is it that some arrow functions use curly braces after the fat arrow and some use parentheses?
For example:

    const foo = (params) =&gt; (
        &lt;span&gt;
            &lt;p&gt;Content&lt;/p&gt;
        &lt;/span&gt;
    );
vs.

    const handleBar = (e) =&gt; {
        e.preventDefault();
        dispatch(&#39;logout&#39;);
    };


Content Type	Original Author	Original Content on Stackoverflow
Question	Milkncookiez	View Question on Stackoverflow
Solution 1 - Javascript	Joe23	View Answer on Stackoverflow
Solution 2 - Javascript	Odeyinka Olubunmi	View Answer on Stackoverflow
Solution 3 - Javascript	sitifensys	View Answer on Stackoverflow

Arrow function "expression expected" syntax error

Javascript Problem Overview

Javascript Solutions

Solution 1 - Javascript

Solution 2 - Javascript

Solution 3 - Javascript

Rails - The system cannot find the path specified

Promise callbacks returning promises

Attributions