Add type-safety to Record class

This commit adds ability to create type-safe Record entities by assigning type to Record's value.
The implementation is based on adding type parameters to types, interfaces, and classes. It is implemented however in a backwards-compatible manner, so that opting in to type safety is optional.

Examples:

// before (unsafe)
const record = new Record(['name'], ['Alice'])
record.get('neam') // no errors
record.toObject().neam[0] // run-time (late) error

// after (safe)
const record = new Record<Person>(['name'], ['Alice'])
record.get('neam') // Error: does not exist
record.toObject().neam[0] // design-time (early) error

Author: parzhitsky

test/types/record.test.ts

@@ -19,15 +19,25 @@
 
 import Record from '../../types/record'
 
+interface Person {
+  name: string
+  age: number
+}
+
 const record1 = new Record(['name', 'age'], ['Alice', 20])
 const record2 = new Record(['name', 'age'], ['Bob', 22], { key: 'value' })
+const record3 = new Record<Person>(['name', 'age'], ['Carl', 24])
+
 const isRecord1: boolean = record1 instanceof Record
 const isRecord2: boolean = record2 instanceof Record
+const isRecord3: boolean = record3 instanceof Record
 
 const record1Keys: string[] = record1.keys
+const record3Keys: Array<keyof Person> = record3.keys
 const record1Length: number = record1.length
 
 const record1Object: object = record1.toObject()
+const record3Object: Person = record3.toObject()
 
 record1.forEach(() => {})
 
@@ -37,11 +47,24 @@ record1.forEach((value: any, key: string) => {})
 
 record1.forEach((value: any, key: string, record: Record) => {})
 
+record3.forEach(
+  (value: string | number, key: 'name' | 'age', record: Record<Person>) => {}
+)
+
+const record3Mapped: [
+  string | number,
+  'name' | 'age',
+  Record<Person>
+][] = record3.map((...args) => args)
+
 const record1Has: boolean = record1.has(42)
 const record2Has: boolean = record1.has('key')
 
-const record1Get1: any = record1.get(42)
-const record2Get1: any = record2.get('key')
+const record1Get1: any = record1.get('name')
+const record2Get1: any = record2.get('age')
+
+const record1Get2: object = record1.get('name')
+const record2Get2: string[] = record2.get('age')
 
-const record1Get2: object = record1.get(42)
-const record2Get2: string[] = record2.get('key')
+const record3Get1: string = record3.get('name')
+const record3Get2: number = record3.get('age')

types/record.d.ts

@@ -17,29 +17,43 @@
  * limitations under the License.
  */
 
-declare type Visitor = (value: any, key: string, record: Record) => void
-
-declare type MapVisitor<T> = (value: any, key: string, record: Record) => T
+declare type Dict<Key extends PropertyKey = PropertyKey, Value = any> = {
+  [K in Key]: Value
+}
 
-declare class Record {
-  keys: string[]
+declare type Visitor<
+  Entries extends Dict = Dict,
+  Key extends keyof Entries = keyof Entries
+> = MapVisitor<void, Entries, Key>
+
+declare type MapVisitor<
+  ReturnType,
+  Entries extends Dict = Dict,
+  Key extends keyof Entries = keyof Entries
+> = (value: Entries[Key], key: Key, record: Record<Entries>) => ReturnType
+
+declare class Record<
+  Entries extends Dict = Dict,
+  Key extends keyof Entries = keyof Entries
+> {
+  keys: Key[]
   length: number
 
   constructor(
-    keys: string[],
+    keys: Key[],
     fields: any[],
     fieldLookup?: { [index: string]: string }
   )
 
-  forEach(visitor: Visitor): void
+  forEach(visitor: Visitor<Entries, Key>): void
 
-  map<T>(visitor: MapVisitor<T>): T[]
+  map<Value>(visitor: MapVisitor<Value, Entries, Key>): Value[]
 
-  toObject(): object
+  toObject(): Entries
 
-  get(key: string | number): any
+  get<K extends Key>(key: K): Entries[K]
 
-  has(key: string | number): boolean
+  has(key: any): key is Key
 }
 
 export default Record