Add Join type (#235)

index.d.ts

@@ -52,6 +52,7 @@ export {ScreamingSnakeCase} from './source/screaming-snake-case';
 export {DelimiterCase} from './source/delimiter-case';
 export {DelimiterCasedProperties} from './source/delimiter-cased-properties';
 export {DelimiterCasedPropertiesDeep} from './source/delimiter-cased-properties-deep';
+export {Join} from './source/join';
 export {Split} from './source/split';
 export {Trim} from './source/trim';
 export {Includes} from './source/includes';

readme.md

@@ -143,6 +143,7 @@ Click the type names for complete docs.
 - [`DelimiterCase`](source/delimiter-case.d.ts) – Convert a string literal to a custom string delimiter casing.
 - [`DelimiterCasedProperties`](source/delimiter-cased-properties.d.ts) – Convert object properties to a custom string delimiter casing.
 - [`DelimiterCasedPropertiesDeep`](source/delimiter-cased-properties-deep.d.ts) – Convert object properties to a custom string delimiter casing recursively.
+- [`Join`](source/join.d.ts) - Join an array of strings using the given string as delimiter.
 - [`Split`](source/split.d.ts) - Represents an array of strings split using a given character or character set.
 - [`Trim`](source/trim.d.ts) - Remove leading and trailing spaces from a string.
 - [`Get`](source/get.d.ts) - Get a deeply-nested property from an object using a key path, like [Lodash's `.get()`](https://lodash.com/docs/latest#get) function.

source/join.d.ts

@@ -0,0 +1,22 @@
+/**
+Join an array of strings using the given string as delimiter.
+
+Use-case: Defining key paths in a nested object. For example, for dot-notation fields in MongoDB queries.
+
+@example
+```
+import {Join} from 'type-fest';
+
+const path: Join<['foo', 'bar', 'baz'], '.'> = ['foo', 'bar', 'baz'].join('.');
+```
+
+@category Template Literals
+*/
+export type Join<
+	Strings extends string[],
+	Delimiter extends string,
+> = Strings extends [] ? '' :
+	Strings extends [string] ? `${Strings[0]}` :
+	// @ts-expect-error `Rest` is inferred as `unknown` here: https://github.com/microsoft/TypeScript/issues/45281
+	Strings extends [string, ...infer Rest] ? `${Strings[0]}${Delimiter}${Join<Rest, Delimiter>}` :
+	string;

test-d/join.ts

@@ -0,0 +1,19 @@
+import {expectError, expectType} from 'tsd';
+import {Join} from '../index';
+
+// General use.
+const generalTest: Join<['foo', 'bar', 'baz'], '.'> = 'foo.bar.baz';
+expectType<'foo.bar.baz'>(generalTest);
+expectError<'foo'>(generalTest);
+expectError<'foo.bar'>(generalTest);
+expectError<'foo.bar.ham'>(generalTest);
+
+// Empty string delimiter.
+const emptyDelimiter: Join<['foo', 'bar', 'baz'], ''> = 'foobarbaz';
+expectType<'foobarbaz'>(emptyDelimiter);
+expectError<'foo.bar.baz'>(emptyDelimiter);
+
+// Empty input.
+const emptyInput: Join<[], '.'> = '';
+expectType<''>(emptyInput);
+expectError<'foo'>(emptyInput);