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?

user_3001761 · Answer

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. 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

> strict-jest@0.1.0 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.

How to disable "Use Strict" in Jest

1 Answers