microsoft/TypeScript · error · TypeError

Object expected

Error message

Object expected

What it means

For `accessor`-kind decorated elements, `__esDecorate` requires a non-undefined decorator return value to be an object (with optional get/set/init). Returning a primitive throws "Object expected".

Source

Thrown at src/compiler/factory/emitHelpers.ts:787

    importName: "__esDecorate",
    scoped: false,
    priority: 2,
    text: `
        var __esDecorate = (this && this.__esDecorate) || function (ctor, descriptorIn, decorators, contextIn, initializers, extraInitializers) {
            function accept(f) { if (f !== void 0 && typeof f !== "function") throw new TypeError("Function expected"); return f; }
            var kind = contextIn.kind, key = kind === "getter" ? "get" : kind === "setter" ? "set" : "value";
            var target = !descriptorIn && ctor ? contextIn["static"] ? ctor : ctor.prototype : null;
            var descriptor = descriptorIn || (target ? Object.getOwnPropertyDescriptor(target, contextIn.name) : {});
            var _, done = false;
            for (var i = decorators.length - 1; i >= 0; i--) {
                var context = {};
                for (var p in contextIn) context[p] = p === "access" ? {} : contextIn[p];
                for (var p in contextIn.access) context.access[p] = contextIn.access[p];
                context.addInitializer = function (f) { if (done) throw new TypeError("Cannot add initializers after decoration has completed"); extraInitializers.push(accept(f || null)); };
                var result = (0, decorators[i])(kind === "accessor" ? { get: descriptor.get, set: descriptor.set } : descriptor[key], context);
                if (kind === "accessor") {
                    if (result === void 0) continue;
                    if (result === null || typeof result !== "object") throw new TypeError("Object expected");
                    if (_ = accept(result.get)) descriptor.get = _;
                    if (_ = accept(result.set)) descriptor.set = _;
                    if (_ = accept(result.init)) initializers.unshift(_);
                }
                else if (_ = accept(result)) {
                    if (kind === "field") initializers.unshift(_);
                    else descriptor[key] = _;
                }
            }
            if (target) Object.defineProperty(target, contextIn.name, descriptor);
            done = true;
        };`,
};

const runInitializersHelper: UnscopedEmitHelper = {
    name: "typescript:runInitializers",
    importName: "__runInitializers",
    scoped: false,

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Return `undefined` or an object `{ get?, set?, init? }` from accessor decorators.
2. Use a decorator specifically written for the accessor kind (check ctx.kind === "accessor").
3. Raise target to a runtime with native decorators to avoid the helper.

Example fix

// before
function acc(v, ctx) { return 42; }            // non-object -> "Object expected"
class A { @acc accessor x = 1; }

// after
function acc(v, ctx) { return { init: () => 2 }; }
class A { @acc accessor x = 1; }

function validateAccessorResult(v: unknown): void {
  if (v !== undefined && (v === null || typeof v !== "object")) {
    throw new TypeError("Accessor decorator must return undefined or an object.");
  }
}

function isAccessorDescriptor(v: unknown): v is { get?: Function; set?: Function; init?: Function } {
  return v === undefined || (typeof v === "object" && v !== null);
}

When it happens

Trigger: An accessor decorator returns a number, string, boolean, or any non-object value (other than undefined).

Common situations: Misunderstanding the accessor-decorator contract; reusing a method/field decorator on an accessor.

Related errors

• Function expected
• Cannot add initializers after decoration has completed
• Private accessor was defined without a getter
• Private accessor was defined without a setter
• Symbol.asyncIterator is not defined.

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/6484ec62d363c772. Report an issue: GitHub.