jest-resolve-no-esm

jest-resolve-no-esm

A jest-resolve fork that optionally disables the self-described "bad version" of Jest's attempt at replicating Node's type resolution algorithm.

Motivation

Jest's ESM support is too incomplete. Specifically:

• There is no jest.requireActual function (e.g. a jest.importActual) for ES modules. This makes using jest.unstable_mockModule too annoying since, among other difficulties, I can't seem to import a module with the goal of modifying only a single of its exports while leaving the rest as they are. With something like jest.spyOn and babel-plugin-explicit-exports-references, this becomes is trivial. Unfortunately...

• There is no jest.spyOn support in ESM mode because, as far as current Jest spying functionality is concerned, ES modules are immutable.

So, the solution is to just transpile everything back to CJS so we can regain our mocking and spying abilities. This has been my solution for a long time and it's been working perfectly fine up until (relatively) recently, where Jest updated its resolution algorithm in an attempt to progress its ESM support.

An unfortunate side-effect of this is that, even when transformIgnorePatterns: [] and transform: { ... } are configured in a Jest config file for full CJS transpilation, jest-resolve will still attempt to load the now-transpiled-CJS-module as if it were ESM just because its file names end in .js and the nearest package.json file contains "type": "module".

This fork allows me to disable ESM loading when process.env.JEST_RESOLVE_NO_ESM is truthy, which avoids false positive "errors" like Must use import to load ES Module: ....

[!NOTE]

For now, .mjs and .mts files will always be loaded as ESM even when using this module. This may be revisited later or, ideally, one day this entire package and other annoying workarounds will be obsoleted because Jest will support modern Node's ability to require ES modules natively.

Installation and Usage

• Install:

npm install --save-dev jest-resolve@npm:jest-resolve-no-esm@30 jest

[!NOTE]

You may need to use the package.json overrides feature for a complete installation. See this package for an example.

• Activate:

JEST_RESOLVE_NO_ESM=true npx jest

Alternatively, you could set process.env.JEST_RESOLVE_NO_ESM = 'true' in a Jest config file or setup file.

For example, unified-utils uses this package to test its ESM-only packages.