Add Create A Non-Empty Array Type as a TypeScript TIL

typescript/create-a-non-empty-array-type.md

@@ -0,0 +1,35 @@
+# Create A Non-Empty Array Type
+
+An array type (e.g. `Array<string>`) by default will allow an empty array. We
+can define a custom array type that does static type checking to ensure arrays
+aren't declared as empty.
+
+To do this, we specify the array type has at least one item in addition to the
+_spread_ of the rest of the array.
+
+```typescript
+type NonEmptyArray<T> = [T, ...T[]];
+```
+
+If we declare a `NonEmptyArray` with no items:
+
+```typescript
+const statuses: NonEmptyArray<string> = [];
+```
+
+We'll get the following type error:
+
+```
+Type '[]' is not assignable to type 'NonEmptyArray<string>'.
+  Source has 0 element(s) but target requires 1.
+```
+
+Whereas as soon as we add at least one item, the type error goes away.
+
+```typescript
+const statuses: NonEmptyArray<string> = ['active'];
+```
+
+Here is a [TS Playground example of this type](https://www.typescriptlang.org/play?#code/C4TwDgpgBAcg9gOwKIFsygIICcsEMQA8AKgHxQC8UA2kQDRQB0TRVAuqwNwBQXAxogGdgUCGlABlYLmABXARAEAuWIlToQ2PISFYAlggDmZSm259BwodLkLl8ZGI058BHfqMVqAcly9gugDcIL04gA).
+
+[source](https://twitter.com/he_zhenghao/status/1583557892480778240)