Memoized Selector

Derived Statehard

Course · Section 10: A Closer Look at Functions · Lecture 144: Closures

Implement `createSelector(fn)` returning a function that caches its last result and only recomputes when called with a different (by reference) argument, like a reselect selector.

Examples

Input: selector doubles state.value; call with s1, s1, then s2

Output: results [10, 10, 20], fn called 2 times

The repeated s1 call hits the cache; only new references recompute.

Sample tests

Input: recomputes only on new reference

Output: [10,10,20,2]

Input: same reference is cached

Output: 1

+ 1 hidden test run on Submit.

Hints

Common pitfalls

Deep-comparing arguments defeats the cheap reference check selectors rely on.

Learning resources

React: Choosing the State Structure

Approach & explanation (try first)

A closure caches the last input and output, recomputing only on a new reference, which is how memoized selectors avoid redundant derived-state work.

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