angular/angular · error · Error

Automatic conversion to ${format} is not supported for respo

Error message

Automatic conversion to ${format} is not supported for response type.

What it means

Thrown by the HttpClient testing backend when the flushed body's runtime type matches none of string/number/object/boolean/array — i.e. `undefined`, a function, a symbol, or a bigint. The usual culprit is `flush(undefined)`: a variable that was never assigned. `null` is fine because `_maybeConvertBody` short-circuits it to a null body (with automatic status 204 when no status is given).

Source

Thrown at packages/common/http/testing/src/request.ts:207

    | (boolean | string | number | Object | null)[],
  format: string = 'JSON',
): Object | string | number | (Object | string | number)[] {
  if (typeof ArrayBuffer !== 'undefined' && body instanceof ArrayBuffer) {
    throw new Error(`Automatic conversion to ${format} is not supported for ArrayBuffers.`);
  }
  if (typeof Blob !== 'undefined' && body instanceof Blob) {
    throw new Error(`Automatic conversion to ${format} is not supported for Blobs.`);
  }
  if (
    typeof body === 'string' ||
    typeof body === 'number' ||
    typeof body === 'object' ||
    typeof body === 'boolean' ||
    Array.isArray(body)
  ) {
    return body;
  }
  throw new Error(`Automatic conversion to ${format} is not supported for response type.`);
}

/**
 * Helper function to convert a response body to a string.
 */
function _toTextBody(
  body: ArrayBuffer | Blob | string | number | Object | (string | number | Object | null)[],
): string {
  if (typeof body === 'string') {
    return body;
  }
  if (typeof ArrayBuffer !== 'undefined' && body instanceof ArrayBuffer) {
    throw new Error('Automatic conversion to text is not supported for ArrayBuffers.');
  }
  if (typeof Blob !== 'undefined' && body instanceof Blob) {
    throw new Error('Automatic conversion to text is not supported for Blobs.');
  }
  return JSON.stringify(_toJsonBody(body, 'text'));

View on GitHub (pinned to 51cb07e980)

Solutions

Flush `null` for empty bodies: `req.flush(body ?? null)` — null is valid and yields a 204
Flush a concrete empty representation (empty string, `{}`, `[]`) if the code under test expects a body
Check the fixture: make sure the variable passed to flush() is actually assigned

Example fix

// before
const body = response?.body; // undefined
req.flush(body); // throws

// after
req.flush(body ?? null); // null body, status 204

Defensive patterns

Strategy: validation

Validate before calling

// Guard against accidentally flushing undefined
if (body === undefined) {
  throw new Error('flush(undefined) is not supported — use null for an empty body');
}
req.flush(body ?? null);

Type guard

const isFlushableBody = (b: unknown): b is string | number | boolean | object | null => b === null || !['undefined', 'function', 'symbol', 'bigint'].includes(typeof b);

Prevention

Normalize with `req.flush(body ?? null)` when the body is conditionally populated
Type mock fixtures explicitly (body: MyDto | null) so undefined cannot sneak in
Flush concrete empty representations ({}, '', []) when the code under test expects a body

When it happens

Trigger: `req.flush(undefined)` in a spec (e.g. `req.flush(response.body)` where `response.body` was never set); flushing a function or bigint value into a json/text request.

Common situations: Optional fixture fields left unset; a helper that returns undefined for an empty payload; refactoring mocks so the body variable is conditionally populated.

Related errors

AI-assisted analysis of angular/angular@51cb07e980 (2026-08-22). Data as JSON: /api/errors/ac2a3c9c6bf1c44c. Report an issue: GitHub.