Dependency

You might say

I just downloaded the project and it reports a missing module. Is the code broken?

External code packages a project declares it will use, usually installed from its manifest by a package managerAfter cloning a project, package.json declares its dependencies and a lockfile records resolved versions; the package manager prepares a local install directory from them. Repositories usually do not commit node_modules. If you see Cannot find module, first check whether dependencies were installed, then start the project as documented and inspect the actual error.

Know first

npm

Dependency

From clone to start: when dependencies enter the project

my-first-page/No node_modules yet

package.jsonRecords which packages are needed

src/

node_modules/Does not exist

Terminal

$ npm start
Error: Cannot find module 'react'
Dependencies are not installed yet

The list is there but not the packages: install before starting

For a missing module, separate three causes: never installed, version mismatch, or a wrong import path—each reports differently.

Why dependencies need a separate install step

The project stores a list, not the packages: The repository usually tracks a manifest and may track a lockfile; the install directory is commonly generated locally rather than committed.

Installing prepares the local dependency directory: The package manager reads the manifest and lockfile, downloads or reuses required packages, and prepares a local install directory. A fresh clone or dependency change usually needs installation as documented by the project.

Errors map to different causes: Cannot find module first suggests dependencies were never installed. If it persists after installing, check the version match and whether the import path is wrong.

You can say this to an AI Agent

The freshly cloned project reports Cannot find module. Run the dependency install command first, confirm every package in the manifest is installed, then start again. If installation fails, explain which package failed and why instead of working around it in business code.

Learn next

1npm→2Semantic Versioning→

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A newly cloned project reports Cannot find module 'react' when running npm start. What should happen next?

A. Ask AI to rewrite the code without importing reactB. Reinstall Node.js because a missing module is an environment problemC. Run npm install first, then start the project again