Pre-study

Compiling Vs. Transpiling

Jest Code Transformation

Jest docs – transformIgnorePatterns

Background

When running unit tests using Jest, get the error as below

Jest encountered an unexpected token

    Jest failed to parse a file. This happens e.g. when your code or its dependencies use non-standard JavaScript syntax, or when Jest is not configured to support such syntax.

    Out of the box Jest supports Babel, which will be used to transform your files into valid JS based on your Babel configuration.

    By default "node_modules" folder is ignored by transformers.

    Here's what you can do:
     • If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/ecmascript-modules for how to enable it.
     • 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/configuration
    For information about custom transformations, see:
    https://jestjs.io/docs/code-transformation

    ...

    SyntaxError: Cannot use import statement outside a module

    > 1 | import axios, { isCancel } from 'axios';
        | ^
      2 | import merge from 'lodash/merge';

      at Runtime.createScriptFromCode (node_modules/jest-runtime/build/index.js:1479:14)

Root cause

When a unit test imports a function from a module that uses ECMAScript Module (ESM) syntax, for example, import axios, { isCancel } from 'axios', which leads to Jest fail to parse because the module (e.g., axios) is located in node_modules and uses modern syntax that Jest does not transform by default.

Solution

Whitelist the axios package so that Jest transforms it, even though it is located in node_modules.

In the Jest configuration file, add axios to transformIgnorePatterns, for example:

...

"transformIgnorePatterns": [
  "/node_modules/(?!(axios)/)"
]

...

Multiple packages?

If there are more than one package need to whitelist, for example, axios and intl-messageformat, can do:

...

 "transformIgnorePatterns": [
      "/node_modules/(?!(axios|intl-messageformat)/)"
    ],

...