Question 1

Why does typeof not throw for undeclared variables in JavaScript?

Accepted Answer

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.

Question 2

How does the JavaScript engine resolve a variable?

Accepted Answer

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.

Question 3

What happens if you assign to an undeclared variable in JavaScript?

Accepted Answer

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.

Question 4

What error does accessing a let variable in the TDZ produce?

Accepted Answer

ReferenceError: Cannot access 'x' before initialization. The variable is in the environment record but marked as uninitialized until the declaration line.

Why does typeof not throw for undeclared variables in JavaScript?

More FAQs in How the JavaScript Engine Resolves Variables

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