Is variable resolution in JavaScript slow because of scope chain walking?
No. The engine resolves variables at parse time when possible, knowing the depth of each variable in the chain. It can access variables directly without walking at runtime. Lexical scope is fast.
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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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