Are ES modules strict mode by default?
Yes. Any file with import or export is automatically in strict mode. You do not need 'use strict'. Top-level this is undefined in modules, not the global object.
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More FAQs in Hoisting in Strict Mode JavaScript
No. Hoisting still happens. var is hoisted with undefined, function declarations with their full body, and let/const sit in the TDZ. Strict mode does not change the hoisting mechanism itself.
It makes related bugs visible. Assigning to an undeclared variable throws ReferenceError instead of creating a global. this is undefined in regular functions instead of the global object. Duplicate parameter names throw.
It throws ReferenceError: x is not defined. In sloppy mode, it would silently create a global variable on the window object, which is a common source of bugs.
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