ES Modules vs CommonJS: The Complete Guide to JavaScript Modules
At some point in your JavaScript journey, you ran into this error:
SyntaxError: Cannot use import statement in a module
Or its annoying cousin:
ReferenceError: require is not defined in ES module scope
These errors mean you've stepped into the module system wars — a fundamental JavaScript ecosystem divide that trips up almost every developer. Let's end the confusion once and for all.
Why Do We Need Modules?
Imagine writing a 10,000-line JavaScript file. You can't. It would be unmaintainable.
Modules let you split your code into separate files, each with its own scope. Variables and functions in one module don't accidentally pollute others. You explicitly declare what you share and what you import. Before modules existed, developers used IIFEs (Immediately Invoked Function Expressions) and global variables. It was messy. Modules solved this properly.
CommonJS (CJS) — Node.js's Original Module System
CommonJS was created for Node.js in 2009. It uses...
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