ReactiveX/rxjs · error · TypeError

MarbleContext.setTimeout: callback must be a function

Error message

MarbleContext.setTimeout: callback must be a function

What it means

The TestScheduler's MarbleContext patches setTimeout to run inside the virtual clock; the callback must be a real function. Passing a string of code (legacy setTimeout('code()') semantics), an object, or undefined throws a TypeError rather than being queued.

Source

Thrown at packages/rxjs/src/testing/index.ts:35

const originalClearTimeout = globalThis.clearTimeout;
const originalSetInterval = globalThis.setInterval;
const originalClearInterval = globalThis.clearInterval;

const flushMicrotasks = Promise.resolve();

export class FakeTimers {
  #timerId = 0;
  #timerQueue: TimerQueueItem[] = [];

  #shouldUseNodeTimeout = typeof globalThis.setTimeout(() => {}) === 'object';

  #setTimeout: typeof globalThis.setTimeout = (() => {
    const patched = (callback: TimerHandler, delay = 0, ...args: any[]): any => {
      const id = ++this.#timerId;
      const time = this.#now + delay;

      if (typeof callback !== 'function') {
        throw new TypeError('MarbleContext.setTimeout: callback must be a function');
      }

      const item: TimerQueueItem = { id, callback: callback as (...args: any[]) => void, delay, time, type: 'timeout', args };
      this.#addTimer(item);

      if (this.#shouldUseNodeTimeout) {
        let ref = false;
        const nodeTimeout = {
          ref: () => {
            ref = true;
            return nodeTimeout;
          },
          unref: () => {
            ref = false;
            return nodeTimeout;
          },
          hasRef: () => {
            return ref;

View on GitHub (pinned to 54796b38a5)

Solutions

Always pass a function: setTimeout(() => doThing(), 100)
Fix optional-callback bugs so undefined is never forwarded to setTimeout
Assert callback type in test setup if the code under test receives timers from untrusted input

Example fix

// before
setTimeout('console.log(1)', 100);
// after
setTimeout(() => console.log(1), 100);

Defensive patterns

Strategy: type-guard

Validate before calling

if (typeof callback !== 'function') throw new TypeError('callback must be a function');
setTimeout(callback, delay);

Type guard

const isFn = (cb: unknown): cb is (...args: any[]) => void => typeof cb === 'function';

Prevention

Never pass strings to setTimeout inside TestScheduler.run
Type callback params as functions, not TimerHandler, in code under test
Guard optional callbacks before forwarding to timers

When it happens

Trigger: Within a TestScheduler.run(...) callback calling setTimeout('doThing()', 100), or passing a non-function callback such as setTimeout(null, 10) or forwarding a possibly-undefined handler.

Common situations: Legacy browser-style code using string callbacks, or application code under test that conditionally calls setTimeout(cb) where cb can be undefined due to an optional callback bug.

Understand the failure class

Timeouts: ETIMEDOUT, deadlines, and hung requests — what actually expires when a request times out.

Related errors

MarbleContext.setInterval: callback must be a function

AI-assisted analysis of ReactiveX/rxjs@54796b38a5 (2026-08-28). Data as JSON: /api/errors/6f54a6fdc28eb69e. Report an issue: GitHub.