Main property in package.json defines package entry point

I know my project's dependencies are installed under node_modules directory. But when I do require('lodash') , how does Node know which file to load?

"How does Node know which file to read when loading a module?"

Module loading works in two phases

When loading a dependency, module loading in Node.js works in two phases. This is in contrast to loading a core node module with require('fs') or a local module with require('./queue/mem.js'). A dependency is loaded in two phases 1) the right directory is looked up and 2) the entry point within that directory is located.

Finding directory

Node looks up all node_modules directories that are on the path from the calling file and the root of the filesystem. It starts from the directory containing the file with require call and works all the way up the directory hierarchy. Node keeps looking until it finds a directory name under node_modules directory matching that of the require call. If no directory provides a hit, Node will try few system directories as a last resort.

Locating entry point

After finding the directory Node tries a couple of strategies to determining the entry point of the package. The entry point is the file that is to be loaded and its exports object to be returned as the return value of the originating require call. First, Node looks for a package.json file and checks if it contains a main property. It will be used to point a file inside the package directory that will be the entry point. If main property does not exist, then Node tries in order index.js, index.json and index.node.

What does entry point look like for popular packages

What is done inside a package, is then up to the module authors. There are a couple of ways to organize the entry point to a package. Here's how some of the popular npm modules do it.

Module Main property in package.json
lodash "lodash.js"
async "lib/async.js"
request "index.js"
underscore "underscore.js"
commander "index"
debug "./node.js"
bluebird "./js/release/bluebird.js"
Entry points of popular npm packages.

It can be seen that many packages define the main property and only few leave it to the default lookup convention. The express and chalk packages are the only ones to rely on index lookup. For request and commander packages it would not be necessary to be stating index as the entry point since it is be the default.

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