How To Build A Module Bundler: Complete Guide To 8 Core Techniques...

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Let's walk through how module bundlers work. I want to show you the important parts by building a simple one together. Think of it like taking a box of separate Lego pieces and connecting them into a single, solid model ready for display. We will focus on eight key methods.

We start with the entry point, usually a file like index.js. Our first job is to find it. The module resolver is our file system detective. It takes a request like ./utils and figures out the exact location of the file on your computer.

The code begins by checking if the path is relative. It joins the path with the directory of the file that asked for it. For a non-relative request like lodash, it has a different job. It walks up the folder tree, looking for a node_modules folder.

When it finds node_modules/lodash, it doesn't stop. It looks inside for a package.json file. This file tells it which file is the main entry, like lib/index.js. If that file doesn't exist, it tries index.js as a backup. It also needs to try adding extensions like .js or .ts if the path doesn't have one.

Once we can find files, we need to understand their relationships. This is the dependency graph. We begin at the entry file, parse it to see what it imports, then find and parse those files, and repeat. The result is a map of every file and what it needs.

To do this, we must read the source code. We need to find the import and export statements. We can't just use simple text search; we need to understand the code's structure. This is where an Abstract Syntax Tree, or AST, comes in.

Think of an AST as a detailed family tree for your code. Each part of your code—a variable, a function call, an import—becomes a node in this tree. We use a tool like Babel's parser to create this tree from raw text.

With the AST, we can reliably traverse it and collect all the dependencies. We write a small visitor that acts like a security guard checking IDs, looking specifically for nodes of type ImportDeclaration.

We have a graph, but it's just data. Our browser can't execute import statements directly from this graph. We need to transform each module's source code into a new form. We will replace modern ES module syntax with a simple function call system our bundle can understand.

Source: Dev.to