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Development Mode

React Native includes some very useful tools for development: remote JavaScript debugging in Chrome, live reload, hot reloading, and an element inspector similar to the beloved inspector that you use in Chrome. It also performs a bunch of validations while your app is running to give you warnings if you're using a deprecated property or if you forgot to pass a required property into a component, for example.
Screenshots of development mode in action
This comes at a cost: your app runs slower in development mode. You can toggle it on and off from XDE. When you switch it, just close and re-open your app for the change to take effect. Any time you are testing the performance of your app, be sure to disable development mode.