ES modules in Node.js: The status quo

The new ECMAScript (ES) modules are not fully compatible with previous language versions, so the JavaScript engine used needs to know for each file whether it is “old” JavaScript code or a “new” module.

For example, the strict mode preferred by many programmers introduced in ECMAScript 5 was once optional and had to be explicitly enabled, while it is always active in ES modules. Thus, the following snippet can be syntactically interpreted as both conventional JavaScript code and as an ES module:

a = 5;

As a classical Node.js module, this is equivalent to global.a = 5, because the variable a was not declared and strict mode was not explicitly activated, therefore a is treated as a global variable. If you try to load the same file as an ES module, you get the error “ReferenceError: a is not defined” because undeclared variables may not be used in strict mode.

Browsers solve the problem of distinction with an extension of the 

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