npm run build or npx run build code example

Example 1: yarn create react app

// Go into the folder where you wish to create your app
yarn create react-app appname

Example 2: new create react app

npx create-react-app my-app
cd my-app
npm start
//Note:npx on the first line is not a typo — 
//it’s a package runner tool that comes with npm 5.2+.