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Iteration Patterns

Four ways to iterate arrays, ordered by preference.

1. Higher-Order Methods

For data transformations — map, filter, reduce. Preferred because they’re declarative, composable, and return new values:

(bind result (->> items
  (:filter is-valid)
  (:map transform)
  (:reduce combine initial)))

2. for-of

For effectful iteration — logging, DOM updates, side effects:

(for-of item items
  (console:log item))

Use for-of when you need to do something with each element rather than produce something from it.

3. for-each

JavaScript’s .forEach — similar to for-of but as a method:

(items:for-each (fn (:any item) (console:log item)))

for-each can’t break or return from the enclosing function. Prefer for-of when you need early exit.

4. for Loop

For index-based access or performance-critical iteration:

(for (let i 0) (< i items:length) (++ i)
  (console:log i (get items i)))

What to Avoid

for-in on arrays — it iterates property keys (strings, not numbers), includes inherited properties, and doesn’t guarantee order. Never use for-in on arrays.