feat(Typescript): Add a more convienient single-type type guard

stencila · Jul 31, 2019 · 0e59220 · 0e59220
1 parent 1b234f5
commit 0e59220
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Showing 2 changed files with 44 additions and 3 deletions.
diff --git a/ts/util/__tests__/guards.test.ts b/ts/util/__tests__/guards.test.ts
@@ -7,7 +7,8 @@ import {
   isInlineEntity,
   isPrimitive,
   typeIs,
-  nodeIs
+  nodeIs,
+  isa
 } from '../guards'
 
 const primitives = [null, true, false, NaN, 2, 'string']
@@ -55,6 +56,31 @@ describe('nodeIs', () => {
     expect(nodeIs(typeMap)({ type: typeMap.someType })).toBe(true))
 })
 
+describe('isa', () => {
+  const person = { type: 'Person' }
+  const para = { type: 'Paragraph', content: [] }
+
+  test('it returns false for undefined types', () => {
+    // This is a compile error too
+    // @ts-ignore
+    expect(isa(person, 'Foo')).toBe(false)
+  })
+
+  test('it returns true for the right type', () => {
+    expect(isa(person, 'Person')).toBe(true)
+    expect(isa(para, 'Paragraph')).toBe(true)
+  })
+
+  test('it returns false for the wrong type', () => {
+    expect(isa(para, 'Person')).toBe(false)
+    expect(isa(null, 'Person')).toBe(false)
+    expect(isa(true, 'Person')).toBe(false)
+    expect(isa(1.0, 'Person')).toBe(false)
+    expect(isa([], 'Person')).toBe(false)
+    expect(isa({ type: 'Foo' }, 'Person')).toBe(false)
+  })
+})
+
 describe('isPrimitive', () => {
   test('returns true for primitive values', () => {
     primitives.map(node => expect(isPrimitive(node)).toBe(true))

diff --git a/ts/util/guards.ts b/ts/util/guards.ts
@@ -4,7 +4,8 @@ import {
   InlineContent,
   ListItem,
   Node,
-  Paragraph
+  Paragraph,
+  Types
 } from '../types'
 import { TypeMap, TypeMapGeneric } from './type-map'
 import {
@@ -52,7 +53,7 @@ export const nodeIs = <T extends Partial<TypeMap | TypeMapGeneric>>(
  * Returns a boolean value and narrows the TypeScript inferred type to
  * the type.
  *
- * @param type The type to test the
+ * @param type The type to test for
  */
 // eslint-disable-next-line
 export const is = <Ts extends Entity>(type: keyof TypeMap<Ts>) => {
@@ -63,6 +64,20 @@ export const is = <Ts extends Entity>(type: keyof TypeMap<Ts>) => {
   return nodeIs(typeMap)
 }
 
+/**
+ * A type guard to determine whether a node is of a particular type.
+ * Returns a boolean value and narrows the TypeScript inferred type to
+ * the type.
+ *
+ * @param type The type to test for
+ */
+export const isa = <K extends keyof Types>(
+  node: Node,
+  type: K
+): node is Types[K] => {
+  return isEntity(node) && node.type === type
+}
+
 /**
  * Type guard to determine whether a node is a primitive type.
  * Returns a boolean value and narrows the TypeScript inferred type.