jestjs/jest · error · Error

Invalid first argument, ${descriptor}. It must be a named cl

Error message

Invalid first argument, ${descriptor}. It must be a named class, named function, number, or string.

What it means

Thrown by `convertDescriptorToString` (convertDescriptorToString.ts:28-30) when the first argument to a `describe`/`it`/`test` block (the block name) cannot be coerced to a string. The function accepts named functions (uses `.name`), numbers, strings, and undefined; everything else - booleans, symbols, objects, arrays, and anonymous functions (whose `.name` is empty) - falls through to the throw.

Source

Thrown at packages/jest-util/src/convertDescriptorToString.ts:28

export default function convertDescriptorToString(
  descriptor: Global.BlockNameLike | undefined,
): string {
  switch (typeof descriptor) {
    case 'function':
      if (descriptor.name) {
        return descriptor.name;
      }
      break;

    case 'number':
    case 'undefined':
      return `${descriptor}`;

    case 'string':
      return descriptor;
  }

  throw new Error(
    `Invalid first argument, ${descriptor}. It must be a named class, named function, number, or string.`,
  );
}

View on GitHub (pinned to f49721c78e)

Solutions

Pass a literal string (or a named function whose `.name` you intend) as the first argument to `describe`/`it`/`test`.
If using a function as the name, give it a name (`function myScope() {}`) so `.name` is non-empty.
Add a guard before the call to coerce unknown descriptor values: `typeof name === 'string' ? name : String(name)`.

Example fix

// before
describe(() => {}, () => { it('works', () => {}); });
// after
describe('my module', () => { it('works', () => {}); });

Defensive patterns

Strategy: validation

Validate before calling

function validName(d) {
  const t = typeof d;
  return t === 'string' || t === 'number' || (t === 'function' && !!d.name);
}
if (!validName(descriptor)) throw new Error('pass a string/number/named-function name');

Type guard

const isValidBlockName = (d: unknown): boolean =>
  typeof d === 'string' || typeof d === 'number' || (typeof d === 'function' && d.name.length > 0);

Prevention

Always pass a literal string as the describe/it name.
If passing a function, name it (`function billing() {}`) - anonymous arrows have empty `.name`.
Avoid spreading objects/booleans into describe names.

When it happens

Trigger: Calling `describe({}, fn)`, `describe(true, fn)`, `describe(Symbol(), fn)`, `describe([], fn)`, or `describe(() => {}, fn)` (anonymous arrow -> `descriptor.name` is `''` -> falsy -> throw). Also `describe(someObject, fn)`.

Common situations: Passing a variable that was expected to be a string but is an object/boolean; using an anonymous arrow as a describe name expecting it to stringify; copy-paste errors where a name was forgotten; destructuring mistakes producing undefined that then gets default-but-typed weirdly.

Related errors

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/0be83599c62c621a.json. Report an issue: GitHub.