How to not put "use strict" everywhere

I'm trying to write some tests for a React app I've been working on, and I figured I would use Jest since it's mentioned in the React docs.

I'm using Webpack and so far I've installed jest-cli, babel-jest, and I included the following configuration in package.json:

"jest": {
  "scriptPreprocessor": "./node_modules/babel-jest",
  "unmockedModulePathPatterns": [
    "./node_modules/react",
    "./node_modules/react-dom"
  ],
}

So, I'm writing the tests for some file foo.js. This file includes some other module bar.js (i.e. const bar = require('./bar');). Unfortunately, when I run jest I get the following error:

SyntaxError: Block-scoped declarations (let, const, function, class) not yet 
supported outside strict mode in file 'js/foo.js'.

So, after some research, I find out I have to include 'use strict'; at the top of foo-test.js. However, for some reason, I still get the same error unless I also include 'use strict'; at the top of foo.js.

So my question is: am I doing something wrong? If not, is there anyway for me to write my tests using Jest without having to write 'use strict'; at the top of all my source files?