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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>];
+```