microsoft/TypeScript · error · TypeError

Cannot use 'in' operator on non-object

Error message

Cannot use 'in' operator on non-object

What it means

Runtime TypeError from the `__classPrivateFieldIn` helper, which lowers `#field in expr`. Before the brand check it guards the receiver: if `receiver === null` or `typeof receiver` is neither `"object"` nor `"function"`, it throws. The native `in` operator has the same requirement, but this message comes from the downlevel helper.

Source

Thrown at src/compiler/factory/emitHelpers.ts:1375

/**
 * Parameters:
 *  @param state — One of the following:
 *      - A WeakMap when the member is a private instance field.
 *      - A WeakSet when the member is a private instance method or accessor.
 *      - A function value that should be the undecorated class constructor when the member is a private static field, method, or accessor.
 *  @param receiver — The object being checked if it has the private member.
 *
 * Usage:
 * This helper is used to transform `#field in expression` to
 *      `__classPrivateFieldIn(<weakMap/weakSet/constructor>, expression)`
 */
const classPrivateFieldInHelper: UnscopedEmitHelper = {
    name: "typescript:classPrivateFieldIn",
    importName: "__classPrivateFieldIn",
    scoped: false,
    text: `
            var __classPrivateFieldIn = (this && this.__classPrivateFieldIn) || function(state, receiver) {
                if (receiver === null || (typeof receiver !== "object" && typeof receiver !== "function")) throw new TypeError("Cannot use 'in' operator on non-object");
                return typeof state === "function" ? receiver === state : state.has(receiver);
            };`,
};

const addDisposableResourceHelper: UnscopedEmitHelper = {
    name: "typescript:addDisposableResource",
    importName: "__addDisposableResource",
    scoped: false,
    text: `
        var __addDisposableResource = (this && this.__addDisposableResource) || function (env, value, async) {
            if (value !== null && value !== void 0) {
                if (typeof value !== "object" && typeof value !== "function") throw new TypeError("Object expected.");
                var dispose, inner;
                if (async) {
                    if (!Symbol.asyncDispose) throw new TypeError("Symbol.asyncDispose is not defined.");
                    dispose = value[Symbol.asyncDispose];
                }
                if (dispose === void 0) {

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Narrow the operand before the `in` check: `typeof o === "object" && o !== null && #x in o`.
2. Type the parameter as `object` (or the declaring class) so primitives are rejected statically.
3. Guard nullish values upstream so the `in` expression only ever sees objects.
4. Raise `target` to ES2022+ to get the native spec error.

Example fix

// before
function branded(o: any) {
  return #x in o;          // throws if o is null/undefined/primitive
}
// after
function branded(o: unknown) {
  return typeof o === "object" && o !== null && #x in o;
}

function isBranded(o: unknown): boolean {
  return typeof o === "object" && o !== null && #x in o;
}

function isObjectLike(o: unknown): o is object { return typeof o === "object" && o !== null; }

When it happens

Trigger: Evaluating `#x in value` where `value` is `null`, `undefined`, or a primitive (number/string/boolean/symbol/bigint), in code compiled with the `__classPrivateFieldIn` helper. Also reached by calling `__classPrivateFieldIn(state, primitive)` directly.

Common situations: User input or parsed values fed into a `#brand in obj` check without coercion; generic utility code that accepts `unknown`/`any` and runs a private-brand test; nullish short-circuits that were forgotten.

Related errors

• Private accessor was defined without a getter
• Cannot read private member from an object whose class did no
• Private method is not writable
• Private accessor was defined without a setter
• Cannot write private member to an object whose class did not

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/5ab5e7e03589fa60. Report an issue: GitHub.