The .mjs file extension is used for ECMAScript JavaScript modules. It signifies that the file should be treated as a module by JavaScript runtimes, both in browsers and in Node.js environments. This is important because modules have different semantics than traditional JavaScript scripts. Modules have their own scope, meaning variables and functions declared within a module are not automatically available in the global scope. They must be explicitly exported to be used by other modules. Similarly, to use code from other modules, you must explicitly import it. This explicit import/export mechanism promotes code organization, reusability, and avoids naming conflicts. Using .mjs allows JavaScript engines to correctly parse and execute the code as a module, enabling features like import and export statements. It's a crucial part of modern JavaScript development, facilitating modular codebases and better dependency management. The use of .mjs also helps to differentiate module files from traditional JavaScript script files (typically using the .js extension), which might be interpreted differently by older JavaScript engines or build tools. This distinction is particularly important in environments where both module and script files coexist.