jestjs/jest · error · Error

Custom export conditions specified but they are not an…

Error message

Custom export conditions specified but they are not an array of strings

What it means

Thrown by the JSDOM environment constructor when `testEnvironmentOptions.customExportConditions` is present but is not an array of strings. The validation checks `Array.isArray(...) && every(isString)`; if it fails, Jest cannot determine which package.json export conditions to use for module resolution.

Solutions

Set `customExportConditions` to an array of strings: `['browser']`.
If using docblock overrides, ensure valid JSON: `@jest-environment-options {"customExportConditions": ["browser"]}`.
Remove the option entirely if you want the default (`['browser']` for jsdom).

Example fix

// before
module.exports = {
  testEnvironment: 'jsdom',
  testEnvironmentOptions: { customExportConditions: 'browser' },
};

// after
module.exports = {
  testEnvironment: 'jsdom',
  testEnvironmentOptions: { customExportConditions: ['browser'] },
};

Defensive patterns

Strategy: type-guard

Validate before calling

// Validate customExportConditions type in config
function validateExportConditions(opts) {
  if ('customExportConditions' in opts) {
    const c = opts.customExportConditions;
    if (!Array.isArray(c) || !c.every(v => typeof v === 'string')) {
      throw new TypeError('customExportConditions must be an array of strings');
    }
  }
}

Type guard

function isStringArray(value: unknown): value is string[] {
  return Array.isArray(value) && value.every(v => typeof v === 'string');
}

Prevention

Always pass customExportConditions as an array of strings.
Use a JSON schema or config validation step in CI.
When using docblock overrides, validate the JSON with a linter.

When it happens

Trigger: Setting `testEnvironmentOptions: { customExportConditions: 'browser' }` (string instead of array) or `{ customExportConditions: { browser: true } }` (object) or `{ customExportConditions: [123] }` (array with non-string elements) in jest.config.js or in a `@jest-environment-options` docblock.

Common situations: Confusing the type — writing a string or object instead of `['browser']`. Copy-pasting a config snippet from a blog that used the wrong type. Docblock override with malformed JSON.

Related errors

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/9941f5d2146fca6d. Report an issue: GitHub.

Appendix: source

Thrown at packages/jest-environment-jsdom-abstract/src/index.ts:150

    };
    global.removeEventListener = function (
      ...args: Parameters<typeof originalRemoveListener>
    ) {
      if (args[0] === 'error') {
        userErrorListenerCount--;
      }
      return originalRemoveListener.apply(this, args);
    };

    if ('customExportConditions' in projectConfig.testEnvironmentOptions) {
      const {customExportConditions} = projectConfig.testEnvironmentOptions;
      if (
        Array.isArray(customExportConditions) &&
        customExportConditions.every(isString)
      ) {
        this._configuredExportConditions = customExportConditions;
      } else {
        throw new Error(
          'Custom export conditions specified but they are not an array of strings',
        );
      }
    }

    this.moduleMocker = new ModuleMocker(global);

    this.fakeTimers = new LegacyFakeTimers({
      config: projectConfig,
      global,
      moduleMocker: this.moduleMocker,
      timerConfig: {
        idToRef: (id: number) => id,
        refToId: (ref: number) => ref,
      },
    });

    this.fakeTimersModern = new ModernFakeTimers({

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