What is a closure, and what does it capture?

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In shortA closure is a function that can use variables from where it was defined and keeps them alive. Swift, Kotlin and JavaScript capture the variables themselves (later changes are visible); Java captures only effectively final values. Captures are also why closures cause retain cycles and leaks.

Model answer

A closure (or lambda) is a function value that can use variables from the scope where it was created:

fun makeCounter(): () -> Int {
    var count = 0
    return { ++count }      // captures count; it lives on after makeCounter returns
}

What is captured differs by language. Swift, Kotlin and JavaScript/TypeScript capture the variable: if it changes later, the closure sees the new value, and the closure can change it. Swift’s capture list ([x]) takes a snapshot instead. Java lambdas may only use effectively final locals, captured by value, so changing state has to live in an object.

Why it matters: a closure keeps everything it captures alive. In Swift, a closure stored on self that uses self is a retain cycle (fix: [weak self]). On the JVM and in JavaScript, a callback registered with something long-lived (a singleton, a timer, a global event) keeps the whole screen alive until it’s removed.

What they're looking for

A plain definition with an example (a counter or a callback), the difference in capture semantics between languages, and awareness of the memory consequences.

Common mistakes

Confusing closures with anonymous classes only.
Not knowing that captured variables live as long as the closure.
Missing the retain cycle ([weak self]) or listener-leak consequence.

What is a closure, and what does it capture?

Model answer

What they're looking for

Common mistakes

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