jest global variable example

Yep. You put the globals in the package.json. For example, here's an excerpt from the default react-native jest configuration:

"jest": {
    "globals": {
       "__DEV__": true,
       "__RCTProfileIsProfiling": false
     },
     ...
},

This will make the variables available globally when the tests are run.

A cleaner way to add globals would be to set "setupFiles": "<rootDir>/private/jest/setup.js" in package.json, and then create a setup.js file that sets global.__DEV__ = true.

This pattern is helpful for making 3rd party libraries available as globals to Jest tests as well (like Backbone, jQuery, lodash, etc.) - eg. global.Backbone = require('backbone'); and so on.

(Re-submitting this as an answer as it was previously just a comment under Michael Helvey's answer.)