Extension methods in typescript (system)

Question

In my angular2 project I'm trying to extend the prototype of the string class using typescript. This is my code:

interface String 
{
    startsWith(s:string);
    contains(s:string);
    containsOr(s1:string, s2:string);
}

String.prototype.startsWith = function (s:string):boolean {
    return this.indexOf (s) === 0;
}
String.prototype.contains = function (s:string):boolean {
    return this.indexOf(s) > 1;
}
String.prototype.containsOr = function (s1:string, s2:string):boolean {
    return this.indexOf(s1) > 1 || this.indexOf (s2) > 1;
}

With this code the project compiles (also the content assist of Visual Studio Code assists me) but at run-time I get a 'contains is not defined'.

What I'm doing wrong?

Thanks a lot

PS: this is my tsconfig:

{
  "compilerOptions": {
    "target": "es5",
    "module": "system",
    "moduleResolution": "node",
    "sourceMap": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "removeComments": false,
    "noImplicitAny": false,
    "outDir": "wwwroot/app/source/"
  },
  "exclude": [
    "node_modules",
    "bower_components",
    "wwwroot",
    "typings/main",
    "typings/main.d.ts"
  ]
}

EDIT

I noticed that importing the js file in index.html it works, but I don't like this approach.

<script src="app/source/utils/extensions.js"></script>

Answer 1

I was able to get it working with no TS errors (1.8.9), Angular2 (2.0.0-beta.12) errors, and working function call using the following template:

tsconfig.json

{
  "compilerOptions": {
  "target": "es5",
  "module": "system",
  "moduleResolution": "node",
  "sourceMap": true,
  "emitDecoratorMetadata": true,
  "experimentalDecorators": true,
  "removeComments": false,
  "noImplicitAny": false
},
"exclude": [
  "node_modules",
  "typings/main",
  "typings/main.d.ts"
  ]
}

Next, create (if one doesn't exist) a global.d.ts file local to your project:

global.d.ts (local to project, not main TS file of same name)

export {}

   declare global {
     interface String {
       calcWidth(): number;
     }
   }

extensions.ts (entire file)

export {}

//don't redefine if it's already there
if (!String.prototype.hasOwnProperty('calcWidth')) {
    String.prototype.calcWidth = function (): number {
      //width calculations go here, but for brevity just return length
      return this.length;
    }
}

Then in your whatever your first System.import(filename) is (mine is main.ts). Just use once:

import './extensions'  //or whatever path is appropriate
... 
...

Now, throughout your app you can use your interface:

var testString = 'fun test';
console.log(testString.calcWidth());

Produces console output:

8

Hope this is helpful.