React Native doctor that can solve your problems

doctor.sh

#!/usr/bin/env bash

# If you dont know what happened just start this script for automatically trying solve your problem.
# Clear all your build folders and dependencies.
# Install dependencies again

platform=$1
platform=$(echo "$platform" | tr '[:upper:]' '[:lower:]') # to lower string

echo "Moving to the root project folder"
cd ..

echo "Remove npm dependencies"
rm -rf node_modules

echo "Clean watchman and npm cache"
watchman watch-del-all
npm cache clean

echo "Install npm dependencies"
npm install

if [[ "$platform" == "android" ]]
then
    cd android
        ./gradlew clean
    cd ..
elif [[ "$platform" == "ios" ]]
then
    cd ios

    echo "Start cleaning ios build"
    rm -rf build

    echo "Start cleaning ios pods"
    rm -f Podfile.lock
    rm -rf Pods

    ##
    # Fix issue with precompile header errors caused by aggressive caching bug in XCode
    # https://stackoverflow.com/questions/19391768/file-has-been-modified-since-the-precompiled-header-was-built
    ##
    echo "Start cleaning XCode DerivedData"
    rm -rf ~/Library/Developer/Xcode/DerivedData/*
    pod cache clean --all

    echo "Cleaning successfully"
    echo "Install and update pods"
    pod update
    pod install --repo-update

    cd ..
fi

# be careful with this option. This can cause errors
#react-native link

Put this script inside root/scripts/{HERE}
Use this only inside script folder like:
cd scripts && ./doctor.sh ios