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content/Typescript/Add global variable to window.md

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+Sometimes you want to add global variable to your `window`. That thing's called [global module augmentation](https://www.typescriptlang.org/docs/handbook/declaration-merging.html#global-augmentation).
+
+Say you need to call `window.doFancyThings()`. For that you should augment global `window` interface in `*.d.ts` file:
+
+```typescript
+declare global {
+  interface Window {
+    doFancyThings: () => void;
+  }
+}
+```
+
+This is useful for declaring global `window.ethereum` (or `window.web3`) in [blockchain](/blockchain/Common%20typescript%20examples) projects with typescript, which use wallet browser extensions. 
+
+## Augmenting existing interface
+
+For example, you have class `Sample` without any functionality:
+
+```typescript
+// Sample.ts
+
+export class Sample {
+  // nothing :-)
+}
+```
+
+Then you want extend it with `doFancyThings()` method. That can be achieved with said [module augmentation](https://www.typescriptlang.org/docs/handbook/declaration-merging.html#module-augmentation):
+
+```typescript
+// fancyThings.ts
+import { Sample } from "./Sample";
+
+declare module "./Sample" {
+  interface Sample {
+    doFancyThings: () => void;
+  }
+}
+```
+
+Now you can call `sample.doFancyThings()` by importing both `.ts` files:
+
+```typescript
+import { Sample } from "./sample";
+import "./fancyThings";
+
+const sample = new Sample();
+sample.doFancyThings(); // ok
+```
+
+This example is useful for [adding global properties to component](../Frontend/Vue/Adding%20global%20properties%20to%20component.md) in vue.js.