Angular2 & Typescript: How to add string extension method/prototype

Question

I want to add extension method format() to String. So my expectation is that I can use String.format wherever in my project. I had followed the guideline of this topic but this not help. I got this error:

Can anyone help me?

Thanks in advance.

p.s: I want to add the extension method like I did in angular 1.xx

---

Edit

use declare global won't get error.

declare global {
interface String {
    format(): string;
}}

String.prototype.format = function () :string {
var result = arguments[0];
for (var i = 0; i < arguments.length - 1; i++) {
    var reg = new RegExp("\\{" + i + "\\}", "gm");
    result = result.replace(reg, arguments[i + 1]);
}
return result;}

How we use String.format('<img alt="{0}" title="{0}" src="{1}" />', name, id); Since format does not require parameters

Answer 1

For me the following worked in an Angular 6 project using TypeScript 2.8.4.

In the typings.d.ts file add:

interface String {
  format(...args: string[]): string;
}

Note: No need to 'declare global'.

In a new file called string.extensions.ts add the following:

interface String {
  format(...args: string[]): string;
}

String.prototype.format = function (...args: string[]): string {
  var s = this;
  return s.replace(/{(\d+)}/g, function (match, number) {
    return (typeof args[number] != 'undefined') ? args[number] : match;
  });
};

To use it, first import it:

import '../../string.extensions';

Obviously your import statement should point to the correct path. Inside your class's constructor or any method:

console.log("Hello {0}".format("world"));

Answer 2

Based on this playground it works just fine.

It probably doesn't work for you because you're probably using modules (import/export), in that case you need to do that in the global augmentation:

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

Then you won't get an error when adding foo to the prototype.

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Edit

Seems like you want a static function on String, so you need to do this:

declare global {
    interface StringConstructor {
        format(): string;
    }
}

String.format = function (...args: string[]) {
    ...
}

I also added the ...args: string[] to the signature which tells the compiler that the function expects any number of strings as arguments.