hexojs/hexo · error · TypeError

fn must be a function

Error message

fn must be a function

What it means

Thrown by Generator.register when no function can be resolved. Hexo generators build the site's route data; the (fn) form requires the single argument to be a function, otherwise an auto-name cannot be assigned. The (name, fn) form also requires fn callable.

Source

Thrown at lib/extend/generator.ts:49

  }

  list(): Store {
    return this.store;
  }

  get(name: string): StoreFunction {
    return this.store[name];
  }

  register(fn: GeneratorFunction): void
  register(name: string, fn: GeneratorFunction): void
  register(name: string | GeneratorFunction, fn?: GeneratorFunction): void {
    if (!fn) {
      if (typeof name === 'function') { // fn
        fn = name;
        name = `generator-${this.id++}`;
      } else {
        throw new TypeError('fn must be a function');
      }
    }

    if (fn.length > 1) fn = Promise.promisify(fn);
    this.store[name as string] = Promise.method(fn);
  }
}

export = Generator;

View on GitHub (pinned to 059cb17494)

Solutions

1. Pass a function as the generator (register(fn)) or as the second argument (register(name, fn)).
2. Verify the imported generator symbol is defined.
3. Ensure you did not pass an options object as the sole argument.

Example fix

// before
hexo.extend.generator.register(genConfig);
// after
hexo.extend.generator.register(locals => { /* return routes */ });

if (typeof fn !== 'function') throw new Error('generator must be a function');
hexo.extend.generator.register(fn);

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

When it happens

Trigger: Calling register('non-function-string') with no fn, or register(someObject) where the argument is not a function.

Common situations: A plugin registers a generator but the handler import is undefined, or an object/config was passed instead of a function.

Related errors

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

AI-assisted analysis of hexojs/hexo@059cb17494 (2026-08-12). Data as JSON: /api/errors/7e8483c9a14fde15. Report an issue: GitHub.