Fix exception on older browsers

lib/callable-instance.js

@@ -30,7 +30,9 @@ export const CallableInstance =
 
         for (const p of names) {
           const descriptor = Object.getOwnPropertyDescriptor(value, p)
-          if (descriptor) Object.defineProperty(apply, p, descriptor)
+          if (descriptor && descriptor.configurable) {
+            Object.defineProperty(apply, p, descriptor)
+          }
         }
 
         return apply

test/callable-instance.js

@@ -0,0 +1,70 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+import {CallableInstance} from '../lib/callable-instance.js'
+
+test('callable-instance', async function (t) {
+  await t.test('can invoke ES6 class', async function () {
+    class Es6Class extends CallableInstance {
+      constructor() {
+        super('copy')
+
+        /** @type {number} */
+        this.foo = 42
+        /** @type {number} */
+        this.bar = 0
+      }
+
+      /** @returns {Es6Class} */
+      copy() {
+        const destination = new Es6Class()
+        destination.foo = this.foo
+        destination.bar = this.bar
+        return destination
+      }
+    }
+
+    const instance = new Es6Class()
+    assert.strictEqual(instance.foo, 42)
+
+    instance.bar = 100
+
+    // Instance is callable
+    /** @type {Es6Class} */
+    const copied = instance()
+    assert.strictEqual(copied.foo, 42)
+    assert.strictEqual(copied.bar, 100)
+  })
+
+  await t.test('can invoke new (ES5 class)', async function () {
+    function Es5Class() {
+      /** @type {Function} */
+      const callableInstance = CallableInstance
+      return callableInstance.call(this, ['copy'])
+    }
+
+    Es5Class.prototype.foo = 42 // type-coverage:ignore-line
+    Es5Class.prototype.bar = 0 // type-coverage:ignore-line
+
+    // type-coverage:ignore-next-line
+    Es5Class.prototype.copy = function () {
+      const destination = new Es5Class()
+      destination.foo = this.foo
+      destination.bar = this.bar
+      return destination
+    }
+
+    // Es5Class is newable
+    const instance = new Es5Class()
+    assert.strictEqual(instance.foo, 42)
+
+    /** @type {number} */
+    instance.bar = 100
+
+    // Instance is callable
+    /** @type {Es5Class} */
+    // @ts-expect-error Es5Class is also callable
+    const copied = instance()
+    assert.strictEqual(copied.foo, 42)
+    assert.strictEqual(copied.bar, 100)
+  })
+})

test/index.js

@@ -1,4 +1,5 @@
 /* eslint-disable import/no-unassigned-import */
+import './callable-instance.js'
 import './core.js'
 import './data.js'
 import './freeze.js'