undefined, JavaScript's Extra Flavor

JavaScript has a second kind of nothing, and this one has no Python equivalent at all.

null means someone deliberately set this to nothing.

undefined means nobody ever set this at all.

A variable you declare but never assign is undefined. An object property that doesn't exist is undefined. A function that returns nothing gives back undefined.

EXAMPLE

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YOUR TURN

Your turn → Fix the Python version: create never_set above the print and give it a value. Then, in JavaScript, give neverSet a value and add an age to person.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

✦ Direct equivalent, similar concept, or language-specific?

Python None ↔ JavaScript null is a SIMILAR CONCEPT, safe to translate. JavaScript undefined is a LANGUAGE-SPECIFIC FEATURE with no Python counterpart. Don't force a false equivalence just to keep the two languages symmetrical. Real understanding means knowing where the map ends.

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