The `::` is [a proposed binding operator][1] that desugars into a bound function:

    ::foo.bar
    // becomes
    foo.bar.bind(foo)

This is useful in React (and any other event handlers) because it means `this` will have the expected value (instance of the class) when the event handler is later invoked.


  [1]: https://github.com/tc39/proposal-bind-operator

I have a data frame like this in pandas:

     column1      column2
     [a,b,c]        1
     [d,e,f]        2
     [g,h,i]        3

**Expected output:**
========================================
    column1      column2
      a              1
      b              1
      c              1
      d              2
      e              2
      f              2
      g              3
      h              3
      i              3

How to process this data ? 

Pandas expand rows from list data available in column

I&#39;m having issues transitioning to pandas from R where `dplyr` package can easily group-by and perform multiple summarizations. 

Please help improve my existing Python pandas code for multiple aggregations:

    import pandas as pd
    data = pd.DataFrame(
        {&#39;col1&#39;:[1,1,1,1,1,2,2,2,2,2],
        &#39;col2&#39;:[1,2,3,4,5,6,7,8,9,0],
         &#39;col3&#39;:[-1,-2,-3,-4,-5,-6,-7,-8,-9,0]
        }
    )
    result = []
    for k,v in data.groupby(&#39;col1&#39;):
        result.append([k, max(v[&#39;col2&#39;]), min(v[&#39;col3&#39;])])
    print pd.DataFrame(result, columns=[&#39;col1&#39;, &#39;col2_agg&#39;, &#39;col3_agg&#39;])

Issues:

 - too verbose
 - probably can be optimized and efficient. (I rewrote a `for-loop groupby` implementation into `groupby.agg` and the performance enhancement was huge). 

In R the equivalent code would be:

    data %&gt;% groupby(col1) %&gt;% summarize(col2_agg=max(col2), col3_agg=min(col3))


-------------
UPDATE: @ayhan solved my question, here is a follow-up question that I will post here instead of as comment: 

Q2) What is the equivalent of `groupby().summarize(newcolumn=max(col2 * col3))` i.e. an aggregation/summarization where the function is a compound function of 2+ columns? 

What is the pandas equivalent of dplyr summarize/aggregate by multiple functions?

I have got some JSX code in a react app like this:

    ...
     _renderSignOutLink() {
        if (!this.props.currentUser) {
          return false;
        }
    
        return (
          &lt;a href=&quot;#&quot; onClick={::this._handleSignOutClick}&gt;&lt;i className=&quot;fa fa-sign-out&quot;/&gt; Sign out&lt;/a&gt;
        );
    ...

What does the double colon, `::`, mean before calling the function?

What does :: (double colon) mean in JavaScript?

I have got some JSX code in a react app like this:
<pre><code class="hljs language-javascript">...
 _renderSignOutLink() {
 if (!this.props.currentUser) {
 return false;
 }

 return (
 &#x3C;a href="#" onClick={::this._handleSignOutClick}>&#x3C;i className="fa fa-sign-out"/> Sign out&#x3C;/a>
 );
...
</code></pre>
What does the double colon, <code>::</code>, mean before calling the function?

How to get current date using the Moment library? Not timestamp, but date. So please don&#39;t refer to that timestamp question already to be found.

I know how to get it in regular javascript, but I need a moment object with current date.

What about timezone, how to get current date in certain timezone?

Moment get current date

My TranslationDetail component is passed an id upon opening, and based on this an external api call is triggered in the class constructor, receiving data to the state, and this data being displayed on TranslationDetail.

    //Routing:
    &lt;Route path=&quot;/translation/:id&quot; component={TranslationDetail}/&gt;
    
    //Class:    
    class TranslationDetail extends Component {
      constructor(props){
        super(props);

        this.props.fetchTrans(this.props.params.id);
      }

This all works fine if I enter the url manually. In case I&#39;d like to use react-router e.g. for displaying the next item like below the url does change, but the api call is not triggered, and the data will remain the same.

    &lt;button 
      type=&quot;button&quot;
      onClick={() =&gt; 
        browserHistory.push(`/translation/${Number(this.props.params.id)+1}`)}&gt;
      Next
    &lt;/button&gt;

Please bear in mind that I&#39;m a total beginner. The reason why this is happening is I believe that the constructor is run only once, thus no further api call is triggered.

How can I solve this?
Do I need to listed to props and call a function on change? If yes, how?

How to fetch data when a React component prop changes?

Test browser: 
Version of Chrome: 52.0.2743.116

It is a simple javascript that is to open an image file from local like &#39;C:\002.jpg&#39;
   
    function run(){

       var URL = &quot;file:///C:\002.jpg&quot;;

       window.open(URL, null);

    }
    run();

Here is my sample code.
https://fiddle.jshell.net/q326vLya/3/

Please give me any suitable suggestions.

Cannot open local file - Chrome: Not allowed to load local resource

It maybe because Sets are relatively new to Javascript but I haven&#39;t been able to find an article, on StackO or anywhere else, that talks about the performance difference between the two in Javascript. So, what is the difference, in terms of performance, between the two? Specifically, when it comes to removing, adding and iterating.

Javascript Set vs. Array performance

I just upgraded to Angular RC 5 and now all component that uses &#39;ROUTER_DIRECTIVES&#39; fails with &#39;Can&#39;t resolve all parameters for Router: (?, ?, ?, ?, ?, ?, ?)&#39; when I try to unit test the component. 


    import { inject, addProviders } from &#39;@angular/core/testing&#39;;
    import { ComponentFixture, TestComponentBuilder } from &#39;@angular/core/testing&#39;;
    import { Component } from &#39;@angular/core&#39;;
    import { ROUTER_DIRECTIVES, Router } from &#39;@angular/router&#39;;
    
    import { HomeComponent } from &#39;./home.component&#39;;
    import { UserService } from &#39;../_services/user.service&#39;;
    
    describe(&#39;Component: Home&#39;, () =&gt; {
    
      beforeEach(() =&gt; {
        addProviders([HomeComponent, UserService, ROUTER_DIRECTIVES, Router]);
      });  
    
      it(&#39;should inject the component&#39;, inject([HomeComponent, UserService, ROUTER_DIRECTIVES, Router],
        (component: HomeComponent) =&gt; {
          expect(component).toBeTruthy();
          // expect(component.currentUser.firstname).toEqual(&#39;Jan&#39;);
        }));

The full error log: 

         Chrome 52.0.2743 (Windows 10 0.0.0)
       Error: Can&#39;t resolve all parameters for Router: (?, ?, ?, ?, ?, ?, ?).
           at new BaseException (webpack:///C:/ng/anbud/~/@angular/compiler/src/facade/exceptions.js:27:0 &lt;- src/test.ts:2943:23)
           at CompileMetadataResolver.getDependenciesMetadata (webpack:///C:/ng/anbud/~/@angular/compiler/src/metadata_resolver.js:551:0 &lt;- src/test.ts:24542:19)
           at CompileMetadataResolver.getTypeMetadata (webpack:///C:/ng/anbud/~/@angular/compiler/src/metadata_resolver.js:448:0 &lt;- src/test.ts:24439:26)
           at webpack:///C:/ng/anbud/~/@angular/compiler/src/metadata_resolver.js:594:0 &lt;- src/test.ts:24585:41
           at Array.forEach (native)
           at CompileMetadataResolver.getProvidersMetadata (webpack:///C:/ng/anbud/~/@angular/compiler/src/metadata_resolver.js:575:0 &lt;- src/test.ts:24566:19)
           at CompileMetadataResolver.getNgModuleMetadata (webpack:///C:/ng/anbud/~/@angular/compiler/src/metadata_resolver.js:305:0 &lt;- src/test.ts:24296:58)
           at RuntimeCompiler._compileComponents (webpack:///C:/ng/anbud/~/@angular/compiler/src/runtime_compiler.js:150:0 &lt;- src/test.ts:37986:47)
           at RuntimeCompiler._compileModuleAndAllComponents (webpack:///C:/ng/anbud/~/@angular/compiler/src/runtime_compiler.js:78:0 &lt;- src/test.ts:37914:37)
           at RuntimeCompiler.compileModuleAndAllComponentsSync (webpack:///C:/ng/anbud/~/@angular/compiler/src/runtime_compiler.js:52:0 &lt;- src/test.ts:37888:21)

Any ideas how to unit test components with routing? 


Can&#39;t resolve all parameters for Router: (?, ?, ?, ?, ?, ?, ?) in Angular RC 5 when unit testing

How to create constants file like: key - value in ReactJs,

    ACTION_INVALID = &quot;This action is invalid!&quot;

and to use that in other components

    errorMsg = myConstClass.ACTION_INVALID;



React create constants file

I have two components:
**Parent Component** from which I want to change child component&#39;s state:

    class ParentComponent extends Component {
      toggleChildMenu() {
        ?????????
      }
      render() {
        return (
          &lt;div&gt;
            &lt;button onClick={toggleChildMenu.bind(this)}&gt;
              Toggle Menu from Parent
            &lt;/button&gt;
            &lt;ChildComponent /&gt;
          &lt;/div&gt;
        );
      }
    }

And **Child Component**:

    class ChildComponent extends Component {
      constructor(props) {
        super(props);
        this.state = {
          open: false;
        }
      }
    
      toggleMenu() {
        this.setState({
          open: !this.state.open
        });
      }
    
      render() {
        return (
          &lt;Drawer open={this.state.open}/&gt;
        );
      }
    }

I need to either change Child Component&#39;s **open** state from Parent Component, or call Child Component&#39;s **toggleMenu()** from Parent Component when Button in Parent Component is clicked?

React js change child component&#39;s state from parent component

I have a react component like :


    import React, { PropTypes, Component } from &#39;react&#39;
    
    
    class MyComponent extends Component {
    
        componentDidMount() {
           window.addEventListener(&quot;beforeunload&quot;, function (event) {
    			console.log(&quot;hellooww&quot;)
    			event.returnValue = &quot;Hellooww&quot;
    		})
        }
    
        componentWillUnmount() {
            window.removeEventListener(&quot;beforeunload&quot;, function (event) {
    			console.log(&quot;hellooww&quot;)
    			event.returnValue = &quot;Hellooww&quot;
    		})
        }
    
        render() {
    
            return (
                &lt;div&gt;
                    Some content
                &lt;/div&gt;
            )
        }
    
    }
    
    export default MyComponent

Here event lister is added to the component. When I refresh the page it gives me pop up asking to leave the page. 

But when I go to another page and do refresh it again shows the same pop-up.

I am removing the `eventListener` from the component on `componentWillUnmount`. Then why it is not being removed?

How can I remove the `beforeunload` event on other pages?

reactjs event listener beforeunload added but not removed

I&#39;m making a website with React, React Router, and Redux. Lots of routes (pages) require users to be logged in. I can redirect to the login page if the user is not logged in like this:

	function requireAuth(nextState, replace) {
		let loggedIn = store.getState().AppReducer.UserReducer.loggedIn;

		if(!loggedIn) {
			replace({
				pathname: &#39;/login&#39;,
				state: {
					nextpathname: nextState.location.pathname
				}
			});
		}
	}

	ReactDOM.render(
		&lt;Provider store={store}&gt;
			&lt;Router history={history}&gt;
				&lt;Route path=&quot;/&quot; component={App}&gt;
					&lt;IndexRoute component={Index} /&gt;
					&lt;Route path=&quot;login&quot; component={Login} /&gt;
					&lt;Route path=&quot;register&quot; component={Register} /&gt;
					&lt;Route path=&quot;dashboard&quot; component={Graph} onEnter={requireAuth}&gt;
						... some other route requires logged in ...
					&lt;/Route&gt;
				&lt;/Route&gt;
			&lt;/Router&gt;
		&lt;/Provider&gt;,
		document.getElementById(&#39;entry&#39;)
	);

Please see the code, I used the onEnter hook to redirect to the &#39;/login&#39; route if the user is not logged in. Data for checking if the user is logged in is in the store and it will update after the user logs in.

It&#39;s working perfectly, but the problem is when I refresh the page, the store is reset and the user is not logged in state back.

I know this happens because the Redux store is just memory storage, so refreshing the page will lose all data from the store.

Checking the server session on every refresh may work but this might be too many requests, so that seems like a bad idea.

Saving the logged in state data to localStorage might work, but in this case, I should check every AJAX calls fail that request rejected because session is expired or not exists like something, and that seems like a bad idea too.

Is there a way to solve this problem more simply? My website needs to handle lots of users so I want to reduce XHR calls as much as possible.

Any advice will be very appreciated.

Content Type	Original Author	Original Content on Stackoverflow
Question	Alexander M.	View Question on Stackoverflow
Solution 1 - Javascript	ssube	View Answer on Stackoverflow

What does :: (double colon) mean in JavaScript?

Javascript Problem Overview

Javascript Solutions

Solution 1 - Javascript

What is the pandas equivalent of dplyr summarize/aggregate by multiple functions?

Pandas expand rows from list data available in column

Attributions