yarn why

Show information about why a package is installed.

yarn why <query>

This command will identify why a package has been installed, detailing which other packages depend upon it, for example, or whether it was explicitly marked as a dependency in the package.json manifest.

yarn why jest 

yarn why vx.x.x [1/4] 🤔 Why do we have the module "jest"...? [2/4] 🚚 Initialising dependency graph... [3/4] 🔍 Finding dependency... [4/4] 🚡 Calculating file sizes... info Has been hoisted to "jest" info This module exists because it's specified in "devDependencies". info Disk size without dependencies: "1.29kB" info Disk size with unique dependencies: "101.31kB" info Disk size with transitive dependencies: "20.35MB" info Amount of shared dependencies: 125 

Query argument

The mandatory query argument for yarn why can be any of:

• a package name (as in the above example)
• a package folder; eg: yarn why node_modules/once
• a file within a package folder; eg: yarn why node_modules/once/once.js

The file locations can also be absolute.