jestjs/jest · error · TypeError

expect.customEqualityTesters: Must be set to an array of Tes

Error message

expect.customEqualityTesters: Must be set to an array of Testers. Was given "${getType(newTesters)}"

What it means

addCustomEqualityTesters (the impl behind expect.addEqualityTesters) requires its argument be an array; it then spreads it into the global customEqualityTesters registry. A non-array would either be iterated incorrectly or silently merged, so Jest validates with Array.isArray and throws TypeError with getType() of the offending value.

Source

Thrown at packages/expect/src/jestMatchersObject.ts:134

      Object.defineProperty(expect.not, key, {
        configurable: true,
        enumerable: true,
        value: (...sample: [unknown, ...Array<unknown>]) =>
          new CustomMatcher(true, ...sample),
        writable: true,
      });
    }
  }

  Object.assign((globalThis as any)[JEST_MATCHERS_OBJECT].matchers, matchers);
};

export const getCustomEqualityTesters = (): Array<Tester> =>
  (globalThis as any)[JEST_MATCHERS_OBJECT].customEqualityTesters;

export const addCustomEqualityTesters = (newTesters: Array<Tester>): void => {
  if (!Array.isArray(newTesters)) {
    throw new TypeError(
      `expect.customEqualityTesters: Must be set to an array of Testers. Was given "${getType(
        newTesters,
      )}"`,
    );
  }

  (globalThis as any)[JEST_MATCHERS_OBJECT].customEqualityTesters.push(
    ...newTesters,
  );
};

View on GitHub (pinned to f49721c78e)

Solutions

Wrap testers in an array: expect.addEqualityTesters([myTester]).
Add multiple at once: expect.addEqualityTesters([testerA, testerB]).
Confirm each entry is a Tester function (signature (a, b, ctx) => boolean | undefined).

Example fix

// before
expect.addEqualityTesters(myDateTester); // single function, not an array

// after
expect.addEqualityTesters([myDateTester]);

Defensive patterns

Strategy: type-guard

Validate before calling

if (!Array.isArray(testers)) {
  throw new TypeError('expect.addEqualityTesters needs an array of Tester functions');
}
expect.addEqualityTesters(testers);

Type guard

function isTesterArray(x: unknown): x is Array<(a: unknown, b: unknown) => boolean | undefined> {
  return Array.isArray(x) && x.every(t => typeof t === 'function');
}

Try / catch

// addCustomEqualityTesters throws synchronously — wrap single testers in an array before calling

Prevention

Always pass an array literal: expect.addEqualityTesters([t]).
Type the parameter as Array<Tester> so TS rejects single functions.
Confirm each entry is a Tester-shaped function before adding.

When it happens

Trigger: Calling expect.addEqualityTesters(myTester) with a single function instead of [myTester], passing null/undefined, or passing an object of testers by mistake.

Common situations: Reading the API as 'add a tester' (singular) and passing one function; copy-paste from a non-array source; migrating from a config where testers were keyed by name.

Related errors

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/269047bcf82a2826.json. Report an issue: GitHub.