Support ".js" endings in import statements

[dospunk]

🐛 Bug Report

When writing import statements, TypeScript allows adding a ".js" ending to a module's name, even when that module is actually a ".ts" file. This is necessary to write modules TypeScript that target the browser, since the browser does not follow Node's module resolution system.

import {thing, otherThing} from "./module.js";
// will not raise an error even though the module we are actually referencing is "./module.ts"

It seems that ts-jest, however, does not allow this. When I try to test a file that includes this style of import, I get an error saying that the module could not be found. If I remove the ".js" ending from the import it runs fine in ts-jest, but then of course it will not work in the browser.

To Reproduce

import a module (ModuleB) into another module (ModuleA) using the style described above, then try to test ModuleA using ts-jest

Expected behavior

I expect the import to behave as it would when running the TypeScript compiler

Link to repo (highly encouraged)

https://github.com/dospunk/ts-jest-bug-repo

Debug log:

# content of ts-jest.log :
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{"context":{"allowJs":false,"logLevel":20,"namespace":"jest-preset","package":"ts-jest","version":"26.4.1"},"message":"creating jest presets not handling JavaScript files","sequence":2,"time":"2020-10-06T19:04:44.955Z"}
{"context":{"logLevel":20,"namespace":"Importer","package":"ts-jest","version":"26.4.1"},"message":"creating Importer singleton","sequence":3,"time":"2020-10-06T19:04:46.895Z"}
{"context":{"allowJs":false,"logLevel":20,"namespace":"jest-preset","package":"ts-jest","version":"26.4.1"},"message":"creating jest presets not handling JavaScript files","sequence":4,"time":"2020-10-06T19:04:46.901Z"}
{"context":{"actualVersion":"26.5.2","expectedVersion":">=26 <27","logLevel":20,"namespace":"versions","package":"ts-jest","version":"26.4.1"},"message":"checking version of jest: OK","sequence":5,"time":"2020-10-06T19:04:46.907Z"}
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envinfo

System:
    OS: Windows 10

Npm packages:
    "@types/jest": "^26.0.14",
    "jest": "^26.5.2",
    "ts-jest": "^26.4.1",
    "typescript": "^4.0.3"

[ahnpnl]

Hi, thank you for reporting the issue. This issue is similar to #1960

I’ve checked your repo and I saw you use outDir to instruct typescript to look for compiled js files. This feature hasn’t been supported yet for ts-jest which is reported in #1960

[ahnpnl]

actually, module not found is caused by jest. Because in this case, the transpile output is var B = require("./ModuleB.js");. ts-jest doesn't do module resolution itself, that is jest does.

Your scenario is you are trying to reference a js file which doesn't exist next to the original ts file, so that's why jest can't find it.

The solution is either:

• Use jest moduleNameMapper to map to https://github.com/dospunk/ts-jest-bug-repo/tree/main/js
• Or create a custom jest module resolution.