Add Join type

readme.md

@@ -140,6 +140,7 @@ Click the type names for complete docs.
 - [`DelimiterCase`](ts41/delimiter-case.d.ts) – Convert a string literal to a custom string delimiter casing.
 - [`DelimiterCasedProperties`](ts41/delimiter-cased-properties.d.ts) – Convert object properties to a custom string delimiter casing.
 - [`DelimiterCasedPropertiesDeep`](ts41/delimiter-cased-properties-deep.d.ts) – Convert object properties to a custom string delimiter casing recursively.
+- [`Join`](ts41/join.d.ts) - Joins an array of strings using a given string as delimiter.
 - [`Split`](ts41/split.d.ts) - Represents an array of strings split using a given character or character set.
 - [`Trim`](ts41/trim.d.ts) - Remove leading and trailing spaces from a string.
 - [`Get`](ts41/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.

test-d/join.ts

@@ -0,0 +1,19 @@
+import {expectError, expectType} from 'tsd';
+import {Join} from '../ts41';
+
+// 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);

ts41/index.d.ts

@@ -18,6 +18,7 @@ export {ScreamingSnakeCase} from './screaming-snake-case';
 export {DelimiterCase} from './delimiter-case';
 export {DelimiterCasedProperties} from './delimiter-cased-properties';
 export {DelimiterCasedPropertiesDeep} from './delimiter-cased-properties-deep';
+export {Join} from './join';
 export {Split} from './split';
 export {Trim} from './trim';
 export {Get} from './get';

ts41/join.d.ts

@@ -0,0 +1,22 @@
+/**
+Joins an array of strings using a given string as delimiter.
+
+Use-case: Defining the type field paths in a nested object for things like 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] ? `${Strings[0]}` :
+	// @ts-expect-error `Rest` is inferred as `unknown` here
+	Strings extends [string, ...infer Rest] ? `${Strings[0]}${Delimiter}${Join<Rest, Delimiter>}` :
+	string;