Practice this fail.

npm would like a word with your flag.

npm / Vite · Restart as often as you like

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why it works. Nothing runs on your machine.

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This trusted local project defines "dev": "vite". Its usual development port is occupied, and you want port 4100.

npm would like a word with your flag.

$ npm run dev

> vite

The usual port is already occupied.

npm would like a word with your flag.

npm run dev -- --port 4100

The -- after the npm script name passes subsequent arguments to that script. Vite’s --port option chooses its listening port, so npm run dev -- --port 4100 supplies the option to Vite rather than consuming it as an npm option.