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This article brings you an in-depth understanding of the NPM mechanism. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.
When using NPM to install, package conflicts often occur (such as inconsistent versions of submodules of multiple main modules, etc.), resulting in various major or minor problems encountered during the development process. All the following contents will be introduced here:
NPM various installation methods
#npm install packageName[@next | @versionNumber]
NPM knows the latest version of each module through the registry query service.
If the current npm project is defined The preinstall hook will be executed at this time.
2. Determine the first-level dependency modulesThe first thing you need to do is to determine the first-level dependencies in the project, that is,
dependencies and ## Modules directly specified in the #devDependencies attribute (assuming no npm install parameters are added at this time).
The project itself is the root node of the entire
dependency tree
If the specified module already exists in the node_modules directory, it will not be reinstalled.
3. Get the module
Getting the module is aThe process of recursion
is divided into the following steps:Get module information
Before downloading a module, you must first determine its version. This is because package.json often contains semantic version (semver, semantic version)
The previous step will get the compressed package address of the module (resolved field). npm will use this address to check the local cache. If it is in the cache, it will be taken directly. If not, it will be downloaded from the warehouse.
If there are dependencies, go back to step 1, if not, stop.
What you get in one step is a complete dependency tree, which may include Lots of repetitive modules. For example, module A depends on loadsh, and module B also depends on lodash. Before npm3, installation was strictly based on the structure of the dependency tree, which resulted in module redundancy.
Starting fromnpm3 version
, a dedupe process has been added by default. It will traverse all nodes and place the modules one by one under the root node, which is the first level of node-modules. When duplicate modules are found, they are discarded.Here you need to define a duplicate module, which refers to
the module name is the same and semver compatible
For example, if the foo module under node-modules depends on lodash@^1.0.0, and the bar module depends on lodash@^1.1.0, then ^1.1.0 is a compatible version.
When foo depends on lodash@^2.0.0 and bar depends on lodash@^1.1.0, according to the rules of semver, there is no compatible version between the two. One version will be placed in node_modules and the other will remain in the dependency tree.-- bar
---- lodash@version2
Assuming that version1 and version2 are compatible versions, after dedupe it will become the following form:
-- foo
Assuming that version1 and version2 are incompatible versions, the subsequent versions are retained in the dependency tree:
node_modules
-- foo-- lodash@version1
-- bar
---- lodash@version2
5 .Install module
This step will update node_modules in the project and execute the life cycle functions in the module (in the order of preinstall, install, postinstall).
6. Execute the project's own life cycle
If the current npm project has a hook defined, it will be executed at this time (in the order of install, postinstall, prepublish, and prepare).
The last step is to generate or update the version description file, and the npm install process is completed.
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