Jest encountered an unexpected token because trying to import a file which Jest cannot parse

PS E:\react\Code\UI> yarn test yarn run v1.17.3 $ jest FAIL src/App.test.js ● Test suite failed to run

Jest encountered an unexpected token

This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.

By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

Here's what you can do:
 • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
 • If you need a custom transformation specify a "transform" option in your config.
 • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

You'll find more details and examples of these config options in the docs:
https://jestjs.io/docs/en/configuration.html

Details:

E:\react\Code\UI\src\mainpage.css:1
({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){body{font-size:12px;font-family:arial,sans-serif;background-size:200% 100%!important;height:100vh}
                                                                                             ^

SyntaxError: Unexpected token {

  1 | import React, { Component } from "react";
> 2 | import "../mainpage.css";
    | ^
  3 | import { connect } from "react-redux";
  4 | import propTypes from "prop-types";
  5 | import { fetchPost } from "../redux/action/postaction";

  at ScriptTransformer._transformAndBuildScript (E:\react../../../../Code/UI/node_modules/@jest/transform/build/ScriptTransformer.js:471:17)
  at ScriptTransformer.transform (E:\react../../../../Code/UI/node_modules/@jest/transform/build/ScriptTransformer.js:513:25)
  at Object.<anonymous> (E:\react../../../../Code/UI/src/page/Header.js:2:1)

Test Suites: 1 failed, 1 total Tests: 0 total Snapshots: 0 total Time: 3.592s Ran all test suites. error Command failed with exit code 1. info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.