hexojs/hexo · error · TypeError

fn must be a function

Error message

fn must be a function

What it means

Thrown by Console.register in the branch where three or four arguments were given, 'options' is truthy, but neither 'options' nor a later argument is a function. Hexo resolves the overloaded signatures (name,desc,fn | name,options,fn | name,desc,options,fn); when the expected fn slot is occupied by a non-function (e.g. an options object) and no real fn follows, the callback cannot be located.

Source

Thrown at lib/extend/console.ts:90

  register(name: string, desc: string, fn: AnyFn): void
  register(name: string, options: Option, fn: AnyFn): void
  register(name: string, desc: string, options: Option, fn: AnyFn): void
  register(name: string, desc: string | Option | AnyFn, options?: Option | AnyFn, fn?: AnyFn): void {
    if (!name) throw new TypeError('name is required');

    if (!fn) {
      if (options) {
        if (typeof options === 'function') {
          fn = options;

          if (typeof desc === 'object') { // name, options, fn
            options = desc;
            desc = '';
          } else { // name, desc, fn
            options = {};
          }
        } else {
          throw new TypeError('fn must be a function');
        }
      } else {
        // name, fn
        if (typeof desc === 'function') {
          fn = desc;
          options = {};
          desc = '';
        } else {
          throw new TypeError('fn must be a function');
        }
      }
    }

    if (fn.length > 1) {
      fn = Promise.promisify(fn);
    } else {
      fn = Promise.method(fn);
    }

View on GitHub (pinned to 059cb17494)

Solutions

1. Ensure the function is the LAST argument in the (name, desc, options, fn) overload.
2. If you only need name+options+fn, put options third and fn fourth.
3. Double-check the overload you intend against the signature order.

Example fix

// before
hexo.extend.console.register('cmd', 'desc', { alias: 'c' });
// after
hexo.extend.console.register('cmd', 'desc', { alias: 'c' }, (args) => { /* ... */ });

const args = [name, desc, options, fn].filter(a => a !== undefined);
if (typeof args[args.length - 1] !== 'function') throw new Error('last argument must be the handler');
hexo.extend.console.register(name, desc, options, fn);

const isFn = (x: unknown): x is (...a: any[]) => any => typeof x === 'function';

When it happens

Trigger: Calling register('name', 'desc', {}) (an options object but no fn), or register('name', { option: 1 }) where the second slot is an object and no function follows.

Common situations: A plugin author intended the (name, desc, fn) form but passed an options object as the third argument and omitted the function, or swapped the options and fn order.

Related errors

• name is required
• fn must be a function
• name is required
• fn must be a function
• type is required

AI-assisted analysis of hexojs/hexo@059cb17494 (2026-08-12). Data as JSON: /api/errors/b969825b5405d268. Report an issue: GitHub.