util: add createClassWrapper to internal/util

Utility function for wrapping an ES6 class with a constructor
function that does not require the new keyword

PR-URL: #11391
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Sam Roberts <[email protected]>

Author: jasnell

lib/internal/util.js

@@ -186,3 +186,23 @@ exports.toLength = function toLength(argument) {
   const len = toInteger(argument);
   return len <= 0 ? 0 : Math.min(len, Number.MAX_SAFE_INTEGER);
 };
+
+// Useful for Wrapping an ES6 Class with a constructor Function that
+// does not require the new keyword. For instance:
+//   class A { constructor(x) {this.x = x;}}
+//   const B = createClassWrapper(A);
+//   B() instanceof A // true
+//   B() instanceof B // true
+exports.createClassWrapper = function createClassWrapper(type) {
+  const fn = function(...args) {
+    return Reflect.construct(type, args, new.target || type);
+  };
+  // Mask the wrapper function name and length values
+  Object.defineProperties(fn, {
+    name: {value: type.name},
+    length: {value: type.length}
+  });
+  Object.setPrototypeOf(fn, type);
+  fn.prototype = type.prototype;
+  return fn;
+};

test/parallel/test-internal-util-classwrapper.js

@@ -0,0 +1,31 @@
+// Flags: --expose-internals
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const util = require('internal/util');
+
+const createClassWrapper = util.createClassWrapper;
+
+class A {
+  constructor(a, b, c) {
+    this.a = a;
+    this.b = b;
+    this.c = c;
+  }
+}
+
+const B = createClassWrapper(A);
+
+assert.strictEqual(typeof B, 'function');
+assert(B(1, 2, 3) instanceof B);
+assert(B(1, 2, 3) instanceof A);
+assert(new B(1, 2, 3) instanceof B);
+assert(new B(1, 2, 3) instanceof A);
+assert.strictEqual(B.name, A.name);
+assert.strictEqual(B.length, A.length);
+
+const b = new B(1, 2, 3);
+assert.strictEqual(b.a, 1);
+assert.strictEqual(b.b, 2);
+assert.strictEqual(b.c, 3);