npm install vs. update – what’s the difference?

The difference between npm install and npm update handling of package versions specified in package.json:

{
  "name":          "my-project",
  "version":       "1.0",                             // install   update
  "dependencies":  {                                  // ------------------
    "already-installed-versionless-module":  "*",     // ignores   "1.0" -> "1.1"
    "already-installed-semver-module":       "^1.4.3" // ignores   "1.4.3" -> "1.5.2"
    "already-installed-versioned-module":    "3.4.1"  // ignores   ignores
    "not-yet-installed-versionless-module":  "*",     // installs  installs
    "not-yet-installed-semver-module":       "^4.2.1" // installs  installs
    "not-yet-installed-versioned-module":    "2.7.8"  // installs  installs
  }
}

Summary: The only big difference is that an already installed module with fuzzy versioning …

• gets ignored by npm install
• gets updated by npm update

Additionally: install and update by default handle devDependencies differently

• npm install will install/update devDependencies unless --production flag is added
• npm update will ignore devDependencies unless --dev flag is added

Why use npm install at all?

Because npm install does more when you look besides handling your dependencies in package.json.
As you can see in npm install you can …

• manually install node-modules
• set them as global (which puts them in the shell’s PATH) using npm install -g <name>
• install certain versions described by git tags
• install from a git url
• force a reinstall with --force