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npm3 Duplication and Deduplication

Let's continue with our example before. Currently we have an application that depends on 2 modules:

our app so far

Now we ask ourselves, what happens if we install another module that depends on Module B v1.0? or Module B v2.0?

Example

Ok, so let's say we want to depend on another package, module D. Module D depends on Module B v2.0, just like Module C.

new module dep, D

Because B v1.0 is already a top-level dependency, we cannot install B v2.0 as a top level dependency. Therefore Module B v2.0 is installed as a nested dependency of Module D, even though we already have a copy installed, nested beneath Module C.

no dedupe

If a secondary dependency is required by 2+ modules, but was not installed as a top-level dependency in the directory hierarchy, it will be duplicated and nested beneath the primary dependency.

However, if a secondary dependency is required by 2+ modules, but is installed as a top-level dependency in the directory hierarchy, it will not be duplicated, and will be shared by the primary dependencies that require it.

For example, let's say we now want to depend on Module E. Module E, like Module A, depends on Module B v1.0.

new module dep, E

Because B v1.0 is already a top-level dependency, we do not need to duplicate and nest it. We simply install Module E and it shares Module B v1.0 with Module A.

dedupe

This appears like this in the terminal:

tree2

Now-- what happens if we update Module A to v2.0, which depends on Module B v2.0, not Module B v1.0?

bump Module A to version 2, deps on Bv2

The key is to remember that install order matters.

Even though Module A was installed first (as v1.0) via our package.json (because it is ordered alphabetically), using the interactive npm install command means that Module A v2.0 is the last package installed.

As a result, npm3 does the following things when we run npm install mod-a@2 --save:

bv1.0 stays even though a doesn't dep on it anymore

This looks like this in the terminal:

tree3

Finally, let's also update Module E to v2.0, which also depends on Module B v2.0 instead of Module B v1.0, just like the Module A update.

bump Module E to version 2, deps on Bv2

npm3 performs the following things:

now we have Bv2.0 everywhere

This looks like this in the terminal:

tree4

Now, this is clearly not ideal. We have Module B v2.0 in nearly every directory. To get rid of duplication, we can run:

npm dedupe

This command resolves all of the packages dependencies on Module B v2.0 by redirecting them to the top level copy of Module B v2.0 and removes all the nested copies.

deduped

This looks like this in the terminal:

tree5

Last modified December 30, 2015           Found a typo? Send a pull request!

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