javascript - How can I mock Webpack's require.context in Jest?

Question

Suppose I have the following module:

var modulesReq = require.context('.', false, /.js$/);
modulesReq.keys().forEach(function(module) {
  modulesReq(module);
});

Jest complains because it doesn't know about require.context:

 FAIL  /foo/bar.spec.js (0s)
● Runtime Error
  - TypeError: require.context is not a function

How can I mock it? I tried using setupTestFrameworkScriptFile Jest configuration but the tests can't see any changes that I've made in require.

Answer 1

I had the same problem, then I've made a 'solution'.

I'm pretty sure that this is not the best choice. I ended up stopping using it, by the points answered here:

https://github.com/facebookincubator/create-react-app/issues/517 https://github.com/facebook/jest/issues/2298

But if you really need it, you should include the polyfill below in every file that you call it (not on the tests file itself, because the require will be no global overridden in a Node environment).

// This condition actually should detect if it's an Node environment
if (typeof require.context === 'undefined') {
  const fs = require('fs');
  const path = require('path');

  require.context = (base = '.', scanSubDirectories = false, regularExpression = /.js$/) => {
    const files = {};

    function readDirectory(directory) {
      fs.readdirSync(directory).forEach((file) => {
        const fullPath = path.resolve(directory, file);

        if (fs.statSync(fullPath).isDirectory()) {
          if (scanSubDirectories) readDirectory(fullPath);

          return;
        }

        if (!regularExpression.test(fullPath)) return;

        files[fullPath] = true;
      });
    }

    readDirectory(path.resolve(__dirname, base));

    function Module(file) {
      return require(file);
    }

    Module.keys = () => Object.keys(files);

    return Module;
  };
}

With this function, you don't need to change any require.context call, it will execute with the same behavior as it would (if it's on webpack it will just use the original implementation, and if it's inside Jest execution, with the polyfill function).