microsoft/TypeScript · error · TypeError

Symbol.iterator is not defined.

Error message

Symbol.iterator is not defined.

What it means

`__values` throws this variant when `Symbol` itself (or Symbol.iterator) is missing from the runtime entirely — a pre-ES2015 environment without a polyfill. The helper distinguishes it from "Object is not iterable." by checking whether `Symbol.iterator` exists.

Source

Thrown at src/compiler/factory/emitHelpers.ts:1047

};

// ES2015 Destructuring Helpers

const valuesHelper: UnscopedEmitHelper = {
    name: "typescript:values",
    importName: "__values",
    scoped: false,
    text: `
            var __values = (this && this.__values) || function(o) {
                var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;
                if (m) return m.call(o);
                if (o && typeof o.length === "number") return {
                    next: function () {
                        if (o && i >= o.length) o = void 0;
                        return { value: o && o[i++], done: !o };
                    }
                };
                throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");
            };`,
};

// ES2015 Generator Helpers

// The __generator helper is used by down-level transformations to emulate the runtime
// semantics of an ES2015 generator function. When called, this helper returns an
// object that implements the Iterator protocol, in that it has `next`, `return`, and
// `throw` methods that step through the generator when invoked.
//
// parameters:
//  @param thisArg  The value to use as the `this` binding for the transformed generator body.
//  @param body     A function that acts as the transformed generator body.
//
// variables:
//  _       Persistent state for the generator that is shared between the helper and the
//          generator body. The state object has the following members:
//            sent() - A method that returns or throws the current completion value.

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Load a Symbol polyfill (e.g. core-js/es6/symbol) before the compiled code runs.
2. Raise `target` to ES2015+ so the helper isn't emitted.
3. Run on a runtime that ships Symbol natively.

Example fix

// before
tsconfig: "target": "es5", "lib": ["es5"]  // old runtime -> "Symbol.iterator is not defined."

// after
require("core-js/es6/symbol");   // polyfill first
// or raise target:
// tsconfig: "target": "es2015"

if (typeof Symbol === "undefined" || !(Symbol as any).iterator) {
  throw new Error("Symbol.iterator missing; load core-js es6.symbol polyfill or raise target to es2015+.");
}

function hasSymbolIterator(): boolean {
  return typeof Symbol !== "undefined" && !!(Symbol as unknown as { iterator?: symbol }).iterator;
}

When it happens

Trigger: Running ES5-targeted TS output on a runtime with no Symbol (old IE, very old Node, embedded JS engines) and no polyfill loaded.

Common situations: target: ES5 with no lib/polyfill; shipping to legacy embedded JS engines.

Related errors

• Symbol.asyncIterator is not defined.
• Object is not iterable.
• Symbol.asyncDispose is not defined.
• Symbol.dispose is not defined.
• Function expected

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/16c0c4c64b5d4b81. Report an issue: GitHub.