Fix test error: "babelHelpers is not defined"

[marilyn79218]

• (Initial) command yarn start works well on master, but yarn test failed for error ReferenceError: babelHelpers is not defined.

Regarding to this thread:

This is because you're using '@babel/plugin-external-helpers', in your plugin list. If you do that, it is up to you to load the helpers. '@babel/plugin-transform-runtime', already loads an alternative approach for helpers, so I would just remove the external helpers plugin.

First step is install new package @babel/plugin-transform-runtime but it will lead to another failure Error: Cannot find module '@babel/core' where it tries to find the latest babel@7 however we are using babel@6.
Hence, we are installing a legacy version of that package [babel-plugin-transform-runtime] to achieve the original purpose.

• The command yarn test works well now!
• But yarn start failed for error:

Error: Runtime helpers are not enabled. Either exclude the transform-runtime Babel plugin or pass the runtimeHelpers: true option. See https://github.com/rollup/rollup-plugin-babel#configuring-babel for more information

Then we found this, where it says

Alternatively, if you know what you're doing, you can use the transform-runtime plugin. If you do this, use runtimeHelpers: true:

rollup.rollup({
  ...,
  plugins: [
    babel({ runtimeHelpers: true })
  ]
}).then(...)

That's it! Then everything works well now.

[marilyn79218]

Also, we are not upgrading babel@6 to babel@7 because there're too many existing packages are legacy itself and they depend on legacy babel@6 as well.

But still, it is very useful to have basic knowledge about babel when you're updating it. Eg, if you're upgrading babel-preset-env to @babel/preset-env, then you will need to update .babelrc accordingly (see thread)

Just like env is now @babel/env, react should be @babel/react and you'll need to install @babel/preset-react.

{
  "presets": [
    [
      "env", // should update to `@babel/env`
      {
        "modules": false
      }
    ],
    "react" // should update to `@babel/react`
  ],
  "env": {
    "test": {
      "presets": [
        [
          "env" // should update to `@babel/env`
        ],
        "react" // should update to `@babel/react`
      ]
    }
  },

[marilyn79218]

Also, if you updated babel-jest@22 to babel-jest@26 in our current project, you will also need to update jest@22 to jest@26 accordingly, see this thread for detail

your reproduction uses jest@23 with babel-jest@24 - that's not supported. Upgrading to jest@24 makes your test pass.

[marilyn79218]

For some experimental upgrades, see this PR.