Function isError

packages/expect/src/jest-utils.ts:90–102

(value: unknown)

// https://github.com/jestjs/jest/blob/905bcbced3d40cdf7aadc4cdf6fb731c4bb3dbe3/packages/expect-utils/src/utils.ts#L509
export function isError(value: unknown): value is Error {
  if (typeof Error.isError === 'function') {
    return Error.isError(value)
  }
  switch (Object.prototype.toString.call(value)) {
    case '[object Error]':
    case '[object Exception]':
    case '[object DOMException]':
      return true
    default:
      return value instanceof Error
  }
};

// Equality function lovingly adapted from isEqual in
//   [Underscore](http://underscorejs.org)