What happens between npm run dev and the page on localhost:5173

The five steps

Every symptom on this page belongs to exactly one of them. Finding the step is most of the work.

• 1. npm resolves the vite binary from node_modules/.bin and runs it.
• 2. Vite loads vite.config.js and merges it with CLI flags.
• 3. The optimizer crawls your entry points and pre-bundles the dependencies it finds.
• 4. The server binds to a port — 5173 unless it is taken or configured otherwise.
• 5. The first request loads the app, and the client opens a WebSocket for HMR.

What the successful start looks like

Vite prints the address block once it is listening. Until that block appears, nothing is serving on 5173.

$ npm run dev

  VITE v8.1.5  ready in 412 ms

  ➜  Local:   http://localhost:5173/
  ➜  Network: use --host to expose
  ➜  press h + enter to show help

Step 1 — npm resolves the binary

The dev script runs vite, and npm looks for it in node_modules/.bin. If the package was never installed, or the install was interrupted, the shell reports a missing command and nothing else happens. This is the failure that never reaches 5173 at all, and it is the most searched one.

Step 2 — the config is read

Vite loads vite.config.js from the project root unless -c points elsewhere. CLI flags win over the config file, which is why a --port on the command line beats server.port. A framework wrapper such as Nuxt or SvelteKit adds a third layer and may override both.

Step 3 — dependencies are pre-bundled

Vite crawls your source for bare imports and converts those packages to ESM once, writing the result to node_modules/.vite. On a cold start this is the slowest step. On later starts it is skipped, because the cache is keyed to the lockfile, the patches directory, the relevant config fields and NODE_ENV — change one of those and the step runs again.

Step 4 — the server binds to a port

Vite listens on 5173 by default and on localhost only. If 5173 is taken, it moves to the next free port and prints the one it actually got, which is why the terminal sometimes says 5174. If host is left at the default, the address answers on your machine and nowhere else.

Step 5 — the HMR socket opens

Once the page loads, the client opens a WebSocket back to the server. Edits travel over that socket. If it fails to connect, the page still renders, but nothing updates until you reload by hand — so a page that loads and never changes is a step-5 problem, not a step-3 one.

Which symptom belongs to which step

• Command not found, or the script exits instantly — step 1.
• Your port or proxy setting appears to be ignored — step 2.
• Long cold start, or a reload right after the first load — step 3.
• The terminal says 5174, or the address answers nowhere but on your machine — step 4.
• The page loads once and never updates again — step 5.
• The page loads and stays blank — none of them: the server answered, so the problem is in your application code.

• When npm run dev does not bring up localhost:5173Read the terminal before the browser: npm run dev fails at one of four points — a missing script, a missing binary, a config error, or a port that is not the one you are opening.
• Dependency pre-bundling on the Vite dev serverVite converts your dependencies to ESM once before serving them on 5173 and caches the result in node_modules/.vite — optimizeDeps controls what goes into that step and what stays out of it.
• Hot Module Replacement on localhost:5173HMR swaps a changed module in the running page over a WebSocket instead of reloading it; when that WebSocket cannot connect, Vite falls back to full page reloads or stops updating at all.
• Clearing the Vite dependency cacheVite caches pre-bundled dependencies in node_modules/.vite — delete that directory or start with --force to rebuild it.
• Changing the port the Vite dev server usesSet server.port in vite.config.js, or pass --port on the command line; add strictPort: true if you want Vite to fail instead of silently moving to the next free port.
• localhost:5173 — the port itselfWhat the address means, common dev ports, and the autocomplete fix.