How to disable "Use Strict" in Jest

Question

I am writing some unit tests for a codebase which uses octal literals. Whenever the test is executed with npm test, a syntax error appears as follows:

Legacy octal literals are not allowed in strict mode.

I should stress that "use strict" does not appear anywhere in the source code, nor can I identify any option in package-lock.json or package.json indicating strict mode. Both JSON files were created with the npm init -y and received no further modification except the addition of:

  "scripts": {
    "test": "jest"
  },

How can I force Jest out of strict mode in order to test code with legacy octal literals?

Answer 1

Per the docs:

By default, Jest will use babel-jest transformer

You can explicitly tell Jest you don't want it to try to apply any transforms by setting the following Jest configuration (e.g. in jest.config.<ext> or under $.jest in the package file):

"transform": {}

See full example below. Alternatively, you can leave Babel's transforms active but configure it with "sourceType": "script".

---

• package.json:

  {
    "name": "strict-jest",
    "version": "0.1.0",
    "description": "",
    "main": "index.js",
    "scripts": {
      "test": "jest"
    },
    "keywords": [],
    "author": "",
    "license": "ISC",
    "devDependencies": {
      "jest": "^28.1.2"
    },
    "jest": {
      "transform": {}
    }
  }
  

• index.test.js:

  it("works", () => {
    expect(0100).toEqual(64);
  });
  

• Output:

  $ npm t
  
  > [email protected] test
  > jest
  
   PASS  ./index.test.js
    ✓ works (2 ms)
  
  Test Suites: 1 passed, 1 total
  Tests:       1 passed, 1 total
  Snapshots:   0 total
  Time:        0.257 s
  Ran all test suites.
  

Answer 2

I also had this issue while working with Jest and TypeScript.

What worked for me was to set "strict": false and "noImplicitUseStrict": true in my tsconfig.json file. I also removed any jest transformations in my jest.config.ts file, like so: "transform": {}