What is the relationship between closures and functional programming in JavaScript?
Closures enable functional programming: currying, partial application, composition, and higher-order functions all rely on closures to capture variables. Without closures, returned functions could not access outer variables, and functional patterns would not work.
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More FAQs in Closures and Higher-Order Functions in JavaScript
When a HOF takes a callback, the callback closes over variables from where it was defined. When a HOF returns a function, the returned function closes over the HOF's local variables. Closures are what make HOFs work.
The callback passed to map closes over variables from where the callback was defined. For example, [1, 2, 3].map(n => n * multiplier) works because the arrow closes over the outer multiplier variable.
compose(f, g) returns a function that closes over f and g. Each call to the returned function invokes f(g(x)). The closure keeps f and g alive so the returned function can use them later.
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