What error does accessing a let variable in the TDZ produce?
ReferenceError: Cannot access 'x' before initialization. The variable is in the environment record but marked as uninitialized until the declaration line.
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More FAQs in How the JavaScript Engine Resolves Variables
It checks the current lexical environment's environment record. If not found, it follows the outer reference to the parent environment and repeats. This continues up to the global scope. If not found, it throws ReferenceError.
In sloppy (non-strict) mode, it creates a global property on window. In strict mode (and ES modules), it throws ReferenceError. Always use strict mode to catch this bug.
It is a spec exception to allow safe feature detection. typeof undeclared returns 'undefined' instead of throwing ReferenceError. This lets you check if a global API exists without crashing.
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