Bundler

A bundler takes an application's entry points, follows every import to build a Module Graph, applies transforms, and emits the files a browser will actually load. It is where module resolution, transpilation, Tree Shaking, Code Splitting, asset handling, and minification all happen.

Bundlers are worth understanding because their behaviour is observable in production but invisible in source. Two bundlers given the same source can differ in ways that matter:

• Import ordering. One may sort a module namespace's keys alphabetically; another preserves source order. Anything downstream that depends on order — a registry, a hash over a list — then differs by build tool rather than by code.
• Loader support. A syntax for importing a file as raw text may exist in one and not another, so a working import becomes an unknown-module error when the build tool changes.
• Chunk grouping. How aggressively client references are merged into shared chunks decides what a page really downloads.

They also complicate development in a specific way: a development server keeps a cache and reloads when its dependency scan finds something new mid-run. That reload aborts in-flight work, which surfaces as a random test file failing on a slow machine and never reproducing on a fast one. See Flaky Test and Race Condition.