Eye Training · Loops and Iteration

For every loop, check four things: where it starts, where it stops, what changes each time, and what's left over at the end.

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J · The basics

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What your eye checks

Whenever you see a loop, ask:

1. Bounds: does it start at the first item and stop at the last? <= against length and starting at length are the classics.
2. What does the loop variable hold? for…in gives keys, for…of gives values, entries() gives both.
3. Does every path move forward? A continue before the increment, or a variable that's never reassigned, means it never ends.
4. Is the collection changed while iterating? splice in a forward loop skips items. (Map and Set deletion during for…of is safe.)
5. What's left over after the loop? A last partial batch, a final buffered line.
6. Does return or break do what's intended? Not inside forEach.
7. Is there I/O inside the loop? One query per item is N+1; sequential awaits multiply latency.
8. What's the cost per pass? includes, find or spread inside a loop makes it quadratic.
9. How long does it run without yielding? A big synchronous loop in a server blocks every other request.
10. Can one item stop the whole loop? Retries without a limit, failures without a dead-letter path.

Tells

Go deeper

• The Event Loop Under Pressure: why one long loop freezes every request
• L3 · Data and SQL: N+1 queries and the fixes in bigger codebases
• Previous surface: Objects and arrays · Next: Functions, this and closures