feat(array): expose ArrayWith type declaration

src/types/ArrayWith.test.ts

@@ -0,0 +1,35 @@
+import { given, then, when } from 'test-fns';
+
+describe('ArrayWith', () => {
+  given('desire for array with min length', () => {
+    then('we can declare the type', () => {
+      let products: ArrayWith<'min', 1, string>;
+    });
+    then('it allows arrays of exactly the min length to it', () => {
+      const products: ArrayWith<'min', 1, string> = ['doughnut']; // 🍩
+    });
+    then('it allows arrays with more than the min length to it', () => {
+      const products: ArrayWith<'min', 1, string> = ['doughnut', 'waffle']; // 🍩, 🧇
+    });
+    then('it blocks arrays with less than the min length to it', () => {
+      // @ts-expect-error; Type '[]' is not assignable to type '[string, ...string[]]'. Source has 0 element(s) but target requires 1.ts(2322)
+      const products: ArrayWith<'min', 1, string> = []; // 🍩, 🧇
+    });
+  });
+  given('desire for array with exact length', () => {
+    then('we can declare the type', () => {
+      let products: ArrayWith<'len', 1, string>;
+    });
+    then('it allows arrays of exactly the desired length to it', () => {
+      const products: ArrayWith<'len', 1, string> = ['doughnut']; // 🍩
+    });
+    then('it allows arrays with more than the desired length to it', () => {
+      // @ts-expect-error; Type '[string, string]' is not assignable to type '[string]'.Source has 2 element(s) but target allows only 1.
+      const products: ArrayWith<'len', 1, string> = ['doughnut', 'waffle']; // 🍩, 🧇
+    });
+    then('it blocks arrays with less than the desired length to it', () => {
+      // @ts-expect-error; Type '[]' is not assignable to type '[string]'. Source has 0 element(s) but target requires 1.ts(2322)
+      const products: ArrayWith<'len', 1, string> = [];
+    });
+  });
+});

src/types/ArrayWith.ts

@@ -0,0 +1,24 @@
+/**
+ * ref:
+ * - https://stackoverflow.com/a/67655609/3068233
+ * - https://stackoverflow.com/a/73409753/3068233
+ */
+type BuildArrayWith<
+  TCheck extends 'len' | 'min', // todo: support 'max' too?
+  TLength extends number,
+  TType,
+  TCurrent extends TType[],
+> = TCurrent['length'] extends TLength
+  ? TCheck extends 'len'
+    ? [...TCurrent]
+    : [...TCurrent, ...TType[]]
+  : BuildArrayWith<TCheck, TLength, TType, [...TCurrent, TType]>;
+
+/**
+ * declares a type of array with a check on its length
+ */
+type ArrayWith<
+  TCheck extends 'len' | 'min', // todo: support 'max' too?
+  TLength extends number,
+  TType,
+> = BuildArrayWith<TCheck, TLength, TType, []>;