Add ReadonlyArray to types

Related-to: syntax-tree/unist-util-visit-parents#17.

Author: wooorm

index.test-d.ts

@@ -1,11 +1,11 @@
-import type {Content, Heading, Paragraph, Root} from 'mdast'
+import type {Heading, Paragraph, RootContent, Root} from 'mdast'
 import {expectAssignable, expectNotType, expectType} from 'tsd'
 import type {Node, Parent} from 'unist'
 import {convert, is} from 'unist-util-is'
 
 // # Setup
 
-const mdastNode = (function (): Root | Content {
+const mdastNode = (function (): Root | RootContent {
   return {type: 'paragraph', children: []}
 })()
 
@@ -49,6 +49,7 @@ if (is(mdastNode, {type: 'heading', depth: 2})) {
 // TS makes this `type: string`.
 if (is(mdastNode, {type: 'paragraph'})) {
   expectNotType<Heading>(mdastNode)
+  expectNotType<Paragraph>(mdastNode)
 }
 
 if (is(mdastNode, {type: 'paragraph'} as const)) {
@@ -111,6 +112,18 @@ if (is(mdastNode, ['heading', isHeading, isHeadingLoose, {type: 'heading'}])) {
   expectNotType<Heading>(mdastNode)
 }
 
+// Can’t narrow down.
+if (is(mdastNode, ['heading'])) {
+  expectNotType<Heading>(mdastNode)
+  expectNotType<Paragraph>(mdastNode)
+}
+
+// Can narrow down with `as const` on arrays of strings.
+if (is(mdastNode, ['heading'] as const)) {
+  expectType<Heading>(mdastNode)
+  expectNotType<Paragraph>(mdastNode)
+}
+
 // # `check`
 
 // No node.

lib/index.js

@@ -1,6 +1,5 @@
 /**
- * @typedef {import('unist').Node} Node
- * @typedef {import('unist').Parent} Parent
+ * @import {Node, Parent} from 'unist'
  */
 
 /**
@@ -28,7 +27,7 @@
  *
  *   Note: `Node` is included as it is common but is not indexable.
  *
- * @typedef {Array<Props | TestFunction | string> | Props | TestFunction | string | null | undefined} Test
+ * @typedef {Array<Props | TestFunction | string> | ReadonlyArray<Props | TestFunction | string> | Props | TestFunction | string | null | undefined} Test
  *   Check for an arbitrary node.
  *
  * @callback TestFunction
@@ -67,6 +66,8 @@ export const is =
   // Note: overloads in JSDoc can’t yet use different `@template`s.
   /**
    * @type {(
+   *   (<Condition extends ReadonlyArray<string>>(node: unknown, test: Condition, index?: number | null | undefined, parent?: Parent | null | undefined, context?: unknown) => node is Node & {type: Condition[number]}) &
+   *   (<Condition extends Array<string>>(node: unknown, test: Condition, index?: number | null | undefined, parent?: Parent | null | undefined, context?: unknown) => node is Node & {type: Condition[number]}) &
    *   (<Condition extends string>(node: unknown, test: Condition, index?: number | null | undefined, parent?: Parent | null | undefined, context?: unknown) => node is Node & {type: Condition}) &
    *   (<Condition extends Props>(node: unknown, test: Condition, index?: number | null | undefined, parent?: Parent | null | undefined, context?: unknown) => node is Node & Condition) &
    *   (<Condition extends TestFunction>(node: unknown, test: Condition, index?: number | null | undefined, parent?: Parent | null | undefined, context?: unknown) => node is Node & Predicate<Condition, Node>) &
@@ -163,7 +164,11 @@ export const convert =
       }
 
       if (typeof test === 'object') {
-        return Array.isArray(test) ? anyFactory(test) : propertiesFactory(test)
+        return Array.isArray(test)
+          ? anyFactory(test)
+          : // Cast because `ReadonlyArray` goes into the above but `isArray`
+            // narrows to `Array`.
+            propertiesFactory(/** @type {Props} */ (test))
       }
 
       if (typeof test === 'string') {