refactor: Rationalize isInstance methods

Author: rekmarks

packages/json-rpc-engine/src/v2/MiddlewareContext.test.ts

@@ -75,14 +75,10 @@ describe('MiddlewareContext', () => {
     );
   });
 
-  it('detects instances of MiddlewareContext', () => {
+  it('identifies instances of MiddlewareContext via isInstance', () => {
     const context = new MiddlewareContext();
-    expect(MiddlewareContext.isInstance(context)).toBe(true);
-  });
 
-  it('detects sub-classes of MiddlewareContext', () => {
-    class SubContext extends MiddlewareContext {}
-    const context = new SubContext();
     expect(MiddlewareContext.isInstance(context)).toBe(true);
+    expect(MiddlewareContext.isInstance({ foo: 'bar' })).toBe(false);
   });
 });

packages/json-rpc-engine/src/v2/MiddlewareContext.ts

@@ -1,6 +1,4 @@
-import { isObject } from '@metamask/utils';
-
-import type { UnionToIntersection } from './utils';
+import { isInstance, type UnionToIntersection } from './utils';
 
 const MiddlewareContextSymbol = Symbol.for('json-rpc-engine#MiddlewareContext');
 
@@ -41,11 +39,7 @@ export class MiddlewareContext<
    * @returns Whether the value is a {@link MiddlewareContext} instance.
    */
   static isInstance(value: unknown): value is MiddlewareContext {
-    return (
-      isObject(value) &&
-      MiddlewareContextSymbol in value &&
-      value[MiddlewareContextSymbol] === true
-    );
+    return isInstance(value, MiddlewareContextSymbol);
   }
 
   constructor(

packages/json-rpc-engine/src/v2/utils.test.ts

@@ -3,6 +3,7 @@ import {
   isNotification,
   stringify,
   JsonRpcEngineError,
+  isInstance,
 } from './utils';
 
 const jsonrpc = '2.0' as const;
@@ -54,6 +55,30 @@ describe('utils', () => {
     });
   });
 
+  describe('isInstance', () => {
+    const TestClassSymbol = Symbol('TestClass');
+
+    class TestClass {
+      private readonly [TestClassSymbol] = true;
+    }
+
+    it('identifies class instances via the symbol property', () => {
+      const value = new TestClass();
+      expect(isInstance(value, TestClassSymbol)).toBe(true);
+    });
+
+    it('identifies plain objects via the symbol property', () => {
+      const value = { [TestClassSymbol]: true };
+      expect(isInstance(value, TestClassSymbol)).toBe(true);
+    });
+
+    it('identifies sub-classes of the class via the symbol property', () => {
+      class SubClass extends TestClass {}
+      const value = new SubClass();
+      expect(isInstance(value, TestClassSymbol)).toBe(true);
+    });
+  });
+
   describe('JsonRpcEngineError', () => {
     it('creates an error with the correct name', () => {
       const error = new JsonRpcEngineError('test');
@@ -62,8 +87,9 @@ describe('utils', () => {
       expect(error.message).toBe('test');
     });
 
-    it('isInstance checks if a value is a JsonRpcEngineError instance', () => {
+    it('identifies JsonRpcEngineError instances via isInstance', () => {
       const error = new JsonRpcEngineError('test');
+
       expect(JsonRpcEngineError.isInstance(error)).toBe(true);
       expect(JsonRpcEngineError.isInstance(new Error('test'))).toBe(false);
     });

packages/json-rpc-engine/src/v2/utils.ts

@@ -1,5 +1,6 @@
 import {
   hasProperty,
+  isObject,
   type JsonRpcNotification,
   type JsonRpcParams,
   type JsonRpcRequest,
@@ -48,6 +49,19 @@ export function stringify(value: unknown): string {
   return JSON.stringify(value, null, 2);
 }
 
+/**
+ * The implementation of static `isInstance` methods for classes that have them.
+ *
+ * @param value - The value to check.
+ * @param symbol - The symbol property to check for.
+ * @returns Whether the value has `{ [symbol]: true }` in its prototype chain.
+ */
+export const isInstance = (
+  value: unknown,
+  symbol: symbol,
+): value is { [key: symbol]: true } =>
+  isObject(value) && symbol in value && value[symbol] === true;
+
 const JsonRpcEngineErrorSymbol = Symbol.for(
   'json-rpc-engine#JsonRpcEngineError',
 );
@@ -67,12 +81,7 @@ export class JsonRpcEngineError extends Error {
    * @param value - The value to check.
    * @returns Whether the value is a {@link JsonRpcEngineError} instance.
    */
-  static isInstance<Value extends Error>(
-    value: Value,
-  ): value is Value & JsonRpcEngineError {
-    return (
-      JsonRpcEngineErrorSymbol in value &&
-      value[JsonRpcEngineErrorSymbol] === true
-    );
+  static isInstance(value: unknown): value is JsonRpcEngineError {
+    return isInstance(value, JsonRpcEngineErrorSymbol);
   }
 }