Allow .slice on Vec types (#3130)

* Allow .slice on Vec types

* Apply suggestions from code review

* Update packages/types/src/codec/AbstractArray.ts

Author: jacogr

packages/types/src/codec/AbstractArray.ts

@@ -159,4 +159,11 @@ export abstract class AbstractArray<T extends Codec> extends Array<T> implements
   public includes (check: unknown): boolean {
     return this.some((value: T) => value.eq(check));
   }
+
+  /**
+   * @description Returns a slice of an array
+   */
+  public slice (start?: number, end?: number): T[] {
+    return this.toArray().slice(start, end);
+  }
 }

packages/types/src/codec/Vec.spec.ts

@@ -6,6 +6,7 @@ import type { Codec, CodecTo } from '../types';
 
 import { Metadata } from '@polkadot/metadata';
 import rpcMetadata from '@polkadot/metadata/static';
+import { randomAsU8a } from '@polkadot/util-crypto';
 
 import { createTypeUnsafe, TypeRegistry } from '../create';
 import { GenericAccountId as AccountId } from '../generic';
@@ -170,5 +171,14 @@ describe('Vec', (): void => {
 
       expect(vec.indexOf(myId)).toEqual(2);
     });
+
+    it('allows a slice operator', (): void => {
+      const vec = registry.createType('Vec<AccountId>', [
+        randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a(), randomAsU8a()
+      ]);
+
+      expect(vec).toHaveLength(10);
+      expect(vec.slice(2, 7)).toHaveLength(5);
+    });
   });
 });