correct implementation

package.json

@@ -21,6 +21,6 @@
   "devDependencies": {
     "@tc39/ecma262-biblio": "2.1.2775",
     "ecmarkup": "20.0.0",
-    "typescript": "5.2.2"
+    "typescript": "5.7.2"
   }
 }

src/index.ts

@@ -1,30 +1,56 @@
-declare var Iterator: {};
-
-if (typeof Iterator === 'undefined' || Iterator == null) {
-  globalThis.Iterator = function() {};
-}
-
-function concatImpl<A>(...iterables: Array<Iterable<A>>): Generator<A>
-function concatImpl(): Generator<never>
-function concatImpl<A>(iterableA: Iterable<A>): Generator<A>
-function concatImpl<A, B>(iterableA: Iterable<A>, iterableB: Iterable<B>): Generator<A | B>
-function concatImpl<A, B, C>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>): Generator<A | B | C>
-function concatImpl<A, B, C, D>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>): Generator<A | B | C | D>
-function concatImpl<A, B, C, D, E>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>, iterableE: Iterable<E>): Generator<A | B | C | D | E>
-function concatImpl(...iterables: Array<Iterable<unknown>>): Generator<unknown>
-function concatImpl(...iterables: Array<unknown>): Generator<unknown> {
-  const openMethods = [];
-  for (let iterable of iterables) {
-    if (iterable == null || Object(iterable) !== iterable) throw new TypeError;
-    let openMethod = (iterable as any)[Symbol.iterator];
+function concatImpl(): Iterator<never>
+function concatImpl<A>(...iterables: Array<Iterable<A>>): Iterator<A>
+function concatImpl<A>(iterableA: Iterable<A>): Iterator<A>
+function concatImpl<A, B>(iterableA: Iterable<A>, iterableB: Iterable<B>): Iterator<A | B>
+function concatImpl<A, B, C>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>): Iterator<A | B | C>
+function concatImpl<A, B, C, D>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>): Iterator<A | B | C | D>
+function concatImpl<A, B, C, D, E>(iterableA: Iterable<A>, iterableB: Iterable<B>, iterableC: Iterable<C>, iterableD: Iterable<D>, iterableE: Iterable<E>): Iterator<A | B | C | D | E>
+function concatImpl(...iterables: Array<Iterable<unknown>>): Iterator<unknown>
+function concatImpl(...iterables: Array<unknown>): Iterator<unknown> {
+  const openMethods: Array<{ openMethod: () => Iterator<unknown>, iterable: Iterable<unknown>}> = [];
+  for (let val of iterables) {
+    if (val == null || Object(val) !== val) throw new TypeError;
+    let iterable: Iterable<unknown> = val as Iterable<unknown>;
+    let openMethod = iterable[Symbol.iterator];
     if (typeof openMethod !== 'function') throw new TypeError;
     openMethods.push({openMethod, iterable});
   }
-  return function*() {
-    for (let { openMethod, iterable } of openMethods) {
-      yield* { [Symbol.iterator]() { return openMethod.call(iterable); } }
-    }
-  }();
+  let done = false;
+  let iterator: Iterator<unknown> | null = null;
+  let nextMethod: Iterator<unknown>["next"] | null = null;
+  return Iterator.from({
+    next() {
+      while (!done) {
+        if (iterator != null && nextMethod != null) {
+          let iterResult = nextMethod.call(iterator);
+          if (!iterResult.done) {
+            return iterResult;
+          }
+          iterator = null;
+          nextMethod = null;
+        }
+        if (openMethods.length > 0) {
+          let { openMethod, iterable } = openMethods.shift()!;
+          let val = openMethod.call(iterable);
+          if (val == null || Object(val) !== val) throw new TypeError;
+          iterator = val as Iterator<unknown>;
+          nextMethod = iterator.next;
+          continue;
+        }
+        done = true;
+      }
+      return { done: true, value: void 0 };
+    },
+    return() {
+      if (!done) {
+        if (iterator != null) {
+          iterator.return?.().value;
+        }
+        done = true;
+      }
+      return { done: true, value: void 0, };
+    },
+  });
 }
 
 // NOTE: this line makes concat non-constructible, and gives it the appropriate name and length

tsconfig.json

@@ -11,7 +11,7 @@
     // "disableReferencedProjectLoad": true,             /* Reduce the number of projects loaded automatically by TypeScript. */
 
     /* Language and Environment */
-    "target": "es2022",                                  /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */
+    "target": "esnext",                                  /* Set the JavaScript language version for emitted JavaScript and include compatible library declarations. */
     // "lib": [],                                        /* Specify a set of bundled library declaration files that describe the target runtime environment. */
     // "jsx": "preserve",                                /* Specify what JSX code is generated. */
     // "experimentalDecorators": true,                   /* Enable experimental support for legacy experimental decorators. */