global function in Ionic 2 / Angular 2

You can easily do that using Providers.When you need to use that functionality (or provider), you just need to inject it into your related component.That is it.

Method 1:

You can create a Provider using CLI.

> ionic g provider YourProvider

your-provider.ts

 import { Injectable } from '@angular/core';

    @Injectable()
    export class YourProvider {

      constructor() {
      }

       openShare() {console.log("button clicked")} 
    }

app.module.ts

import { YourProvider } from "../pages/path";

    @NgModule({
      declarations: [
        MyApp,
      ],
      imports: [
        IonicModule.forRoot(MyApp),
       ],
      bootstrap: [IonicApp],
      entryComponents: [
        MyApp,
        ],
      providers: [{ provide: ErrorHandler, useClass: IonicErrorHandler },YourProvider  ]
    })

    export class AppModule { }

your-view.ts

       import { YourProvider } from '../../providers/your-provider';

        export class YourViewPage{

          constructor(public yourProvider : YourProvider) {

          }

        openShare(){
           this.yourProvider.openShare(); 
        }

Method 2: Creat an abstract base class.

my-base-class.ts

export abstract class MyBaseClass {

  constructor() {
  }

  protected openShare():void {
    console.log("button clicked"); 
  }

}

my-view.ts

export class MyViewPage extends MyBaseClass {
  constructor()
  {
     super();
  }

       openShare(){
               super.openShare(); 
            }
  }

Just like @Sampath mentioned, one way would be to use a custom provider. But since you method is just a simple one, I think you can use Events instead. It'd be like this:

In your app.component.ts file, subscribe to the new event:

import { Events } from 'ionic-angular';

constructor(public events: Events, ...) {

  // ...

  events.subscribe('social:share', () => {

    // your code...
    console.log("button clicked");

  });

}

And then in any other page, just publish the event to execute that logic:

function anotherPageMethod() {
  this.events.publish('social:share');
}

You can use Directive.

Try as follows.

Put the following code in separate directive file. (social-sharing.directive.ts);

import { Directive, HostListener } from '@angular/core';

@Directive({
  selector: '[socialSharing]'
})
export class SocialSharing {

  constructor() { }

  @HostListener('click') onClick() {
    // Your click functionality
  }
}

Import it into app.module.ts and than add to declarations.

In HTML, just add attribute to any element which is the selector in your directive file.

<button ion-button right socialSharing>
 <ion-icon name="md-share"></ion-icon>
</button>

Additional Info:

Passing values from component to directive.

home.html

  <button ion-button right [socialSharing]="property">
    <ion-icon name="md-share"></ion-icon>
  </button>

home.component.ts

 export class HomePage {

   property: string = 'some url';
    constructor() {}
 }

social-sharing.directive.ts

Import Input along with others from @angular/core.

import { Directive, Input, HostListener } from '@angular/core';

@Directive({
   selector: '[socialSharing]'
})

export class SocialSharing {
  @Input('socialSharing') str: string;

  constructor() {}

  @HostListener('click') onClick() {
     console.log(this.str);
  }
};

  ngAfterInit() {
     console.log(this.str);
  };
}

Using Element in Directive:

social-sharing.directive.ts

Import ElementRef along with others from @angular/core

 import { Directive, ElementRef, HostListener } from '@angular/core';

 @Directive({
   selector: '[socialSharing]'
 })

 export class SocialSharing {

   // Add ElementRef to constructor

   constructor(el: ElementRef) {};

   ngOnInit() {
      let elRef = this.el.nativeElement;
      console.log(elRef.innerHTML);        
   };
 }