support serialization in different contexts

spec/regression/issues/138.test.ts

@@ -0,0 +1,13 @@
+import test from 'ava'
+
+import { Substitute } from '../../../src'
+
+interface Library { }
+
+test('issue 138: serializes to JSON compatible data', t => {
+  const lib = Substitute.for<Library>()
+  const result = JSON.stringify(lib)
+
+  t.true(typeof result === 'string')
+  t.is(result, '"@Substitute {\\n}"')
+})

spec/regression/issues/27.test.ts

@@ -0,0 +1,39 @@
+import test from 'ava'
+import { types } from 'util'
+
+import { Substitute } from '../../../src'
+
+interface Library {
+  subSection: () => string
+}
+
+// Adapted snipped extracted from https://github.com/angular/angular/blob/main/packages/compiler/src/parse_util.ts#L176
+// This is to reproduce the behavior of the Angular compiler. This function tries to extract the id from a reference.
+const identifierName = (compileIdentifier: { reference: any } | null | undefined): string | null => {
+  if (!compileIdentifier || !compileIdentifier.reference) {
+    return null
+  }
+  const ref = compileIdentifier.reference
+  if (ref['__anonymousType']) {
+    return ref['__anonymousType']
+  }
+}
+
+test('issue 27: mocks should work with Angular TestBed', t => {
+  const lib = Substitute.for<Library>()
+  lib.subSection().returns('This is the mocked value')
+  const result = identifierName({ reference: lib })
+
+  t.not(result, null)
+  t.true(types.isProxy(result))
+
+  const jitId = `jit_${result}`
+  t.is(jitId, 'jit_property<__anonymousType>: ')
+})
+
+test('issue 27: subsitute node can be coerced to a primitive value', t => {
+  const lib = Substitute.for<Library>()
+  t.true(typeof `${lib}` === 'string')
+  t.true(typeof (lib + '') === 'string')
+  t.is(`${lib}`, '@Substitute {\n}')
+})

src/SubstituteNode.ts

@@ -14,7 +14,7 @@ const clearTypeToFilterMap: Record<ClearType, FilterFunction<SubstituteNodeModel
   substituteValues: node => is.CONTEXT.substitution(node.context)
 }
 
-type SpecialProperty = typeof instance | typeof inspect.custom | 'then'
+type SpecialProperty = typeof instance | typeof inspect.custom | typeof Symbol.toPrimitive | 'then' | 'toJSON'
 type RootContext = { substituteMethodsEnabled: boolean }
 
 export class SubstituteNode extends SubstituteNodeBase implements ObjectSubstitute<unknown>, SubstituteNodeModel {
@@ -259,14 +259,16 @@ export class SubstituteNode extends SubstituteNodeBase implements ObjectSubstitu
   }
 
   private isSpecialProperty(property: PropertyKey): property is SpecialProperty {
-    return property === SubstituteNode.instance || property === inspect.custom || property === 'then'
+    return property === SubstituteNode.instance || property === inspect.custom || property === Symbol.toPrimitive || property === 'then' || property === 'toJSON'
   }
 
   private evaluateSpecialProperty(property: SpecialProperty) {
     switch (property) {
       case SubstituteNode.instance:
         return this
+      case 'toJSON':
       case inspect.custom:
+      case Symbol.toPrimitive:
         return this.printableForm.bind(this)
       case 'then':
         return