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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.

content/Typescript/Flatten object with periods.md

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+This helper generates Typescript types for i18n dictionary json
+files by flattening it with period delimiter. Supports plural forms.
+
+Used for typing [i18n.js](https://www.npmjs.com/package/i18n-js) dictionaries;
+
+```typescript
+import en from './en.json';
+type TranslationPath = Flatten<typeof en>;
+
+const t = (key: TranslationPath, options?: TranslateOptions) =>
+    I18nLib.t(key, options);
+```
+
+Flatten type defined here:
+
+```typescript
+// This one based on answer from StackOverflow:
+// https://stackoverflow.com/questions/58434389/typescript-deep-keyof-of-a-nested-object
+
+export type Flatten<T, D extends number = 5> = [D] extends [never]
+  ? never
+  : T extends PluralForm // plural object
+  ? ''
+  : T extends object
+  ? { [K in keyof T]-?: Join<K, Flatten<T[K], Prev[D]>> }[keyof T]
+  : '';
+
+// Fix it for you plural form
+type PluralForm = Record<'one' | 'few' | 'many', string>;
+
+type Join<K, P> = K extends string | number
+  ? P extends string | number
+    ? `${K}${'' extends P ? '' : '.'}${P}`
+    : never
+  : never;
+
+type Prev = [never, 0, 1, 2, 3, 4, 5, ...Array<0>];
+```

content/Typescript/Type guards.md

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+Useful for type checking at compile and run time:
+
+```typescript
+function isFish(pet: Fish | Bird): pet is Fish {
+  return (pet as Fish).swim !== undefined;
+}
+```
+
+Usage:
+
+```typescript
+const pet = getSmallPet();
+ 
+if (isFish(pet)) {
+  pet.swim();
+} else {
+  pet.fly();
+}
+```
+