microsoft/TypeScript · error · TypeError

Object is not iterable.

Error message

Object is not iterable.

What it means

`__values` downlevels for-of/spread. If the value has neither Symbol.iterator nor a numeric `.length`, and `Symbol.iterator` *does* exist on the runtime, it throws "Object is not iterable." — i.e. the runtime supports iteration but the value isn't iterable.

Solutions

1. Ensure the value is iterable (Array, Map, Set, generator, etc.) before iterating.
2. Add a null/iterable guard around the loop.
3. For plain objects use Object.entries/keys/values instead of for-of on the object.

Example fix

// before
const obj = { a: 1 };
for (const x of obj) { }   // throws "Object is not iterable."

// after
for (const x of Object.entries(obj)) { }
// or guard:
if (obj && obj[Symbol.iterator]) for (const x of obj) { }

function assertIterable(v: unknown): void {
  if (v == null || (typeof (v as any)[Symbol.iterator] !== "function" && typeof (v as any).length !== "number")) {
    throw new TypeError("Object is not iterable.");
  }
}

function isIterable<T>(v: unknown): v is Iterable<T> {
  return v != null && typeof (v as any)[Symbol.iterator] === "function";
}

When it happens

Trigger: for-of (or spread/destructuring) over null, undefined, a plain object without an iterator, or any non-iterable non-array-like value.

Common situations: Iterating an object literal expecting entries; a function returning null where an iterable was expected; spreading a Map without calling .values()/.entries().

Related errors

• Symbol.iterator is not defined.
• Cannot add initializers after decoration has completed
• Cannot read private member from an object whose class did…
• Cannot use 'in' operator on non-object
• Cannot write private member to an object whose class did…

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/496aba346bb5e282. Report an issue: GitHub.

Appendix: 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.

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