tests for init

test/init.test.ts

@@ -0,0 +1,24 @@
+import { expectType } from 'tsd';
+import { init } from '../es';
+
+// string always return string
+expectType<string>(init('abc'));
+// emptyString still returns type string. this is due to `''.chartAt(0) => ''`
+expectType<string>(init(''));
+
+// array literals will read the first type correctly
+expectType<[string, number]>(init(['fi', 1, 'fum']));
+// but if the array is typed as an `Array<T> or T[]`, then return type will be `T`
+expectType<Array<string | number>>(init(['fi', 1, 'fum'] as Array<string | number>));
+// empty array literals return never
+expectType<never>(init([]));
+// but if it is typed, it will be `number[]`
+expectType<number[]>(init([] as number[]));
+// single entry tuples return never, since they literally have no init
+expectType<never>(init([10] as const));
+// tuples return the example type of the input tuple minus the first entry
+expectType<[10, '10']>(init([10, '10', 10] as const));
+expectType<['10', 10]>(init(['10', 10, '10'] as const));
+// typed arrays return the same type
+expectType<Array<number | string>>(init([10, 'ten'] as Array<number | string>));
+expectType<Array<string | number>>(init(['10', 10] as Array<string | number>));