Dependencies

Dependencies are external packages of code your project uses to function.

In the Node.js ecosystem, developers use package.json at the root of the project to declare a list of direct dependencies in the dependencies (e.g., "express": "^4.18.0"). Each of those most likely have transitive dependencies (i.e., express pulls in body-parser, accepts, cookie respectively.). This list of packages, and their dependencies are what constitutes a dependency tree for a project.

Direct vs. transitive

Type	What it is	Example
Direct	Listed in your package.json	express@^4.18.0
Transitive	A dependency of one of your dependencies	body-parser (express needs it)

For example, after running vlt install express typescript, this is what your filesystem might look like:

• package.json list express@^4.18.0 directly
• Directorynode_modules

  • Directoryexpress ← direct dependency (you installed it)

    • index.js
    • Directorynode_modules

      • body-parser ← transitive (express needs it)
      • accepts ← transitive
      • cookie ← transitive
      • …
  • typescript ← direct dependency (dev)

Dependency types

Packages in package.json can be declared under several categories:

Field in package.json	Purpose
dependencies	Production dependencies needed at runtime
devDependencies	Development-only tools (testing, building)
peerDependencies	Expected host dependency (e.g., a component library that is built off react)
optionalDependencies	Non-failing — install skipped if resolution fails

Familiar with CSS selectors? Then you’ll like DSS.

The vlt CLI provides handy colon-prefixed commands to query and filter by dependency types pseudo-state selectors like :prod, :dev, :peer, and :optional.