Here, we’re creating a primitive clone of React’s useState hook. log ( foo ( ) ) // logs 1 - new initialValue, despite exact same call SetFoo ( 1 ) // sets _val inside useState's scopeĬonsole. log ( foo ( ) ) // logs 0 - the initialValue we gave Var = useState ( 0 ) // using array destructuringĬonsole. Return // exposing functions for external use _val = newVal // setting _val without exposing _val Return _val // state() uses _val, declared by parent funciton Var _val = initialValue // _val is a local variable created by useState Let’s look at a practical example to better illustrate this: // Example 0 They’re obviously closely tied to the concept of lexical scoping, which MDN defines as "how a parser resolves variable names when functions are nested”. Kyle Simpson of You Don’t Know JS fame defines closures as such:Ĭlosure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope. In spite of this, they are notorious for being confusing to many especially newer developers. As Mark Dalgleish memorably summarized:Ĭlosures are a fundamental concept in JS. Instead of worrying about bound context, we now have to worry about closures. However, with hooks, some feel we may have swapped one problem for another. One of the many selling points of using hooks is to avoid the complexity of classes and higher order components altogether. Don’t worry, it’s not that hard! What are Closures? It might just help your JS fundamentals if you go through this exercise. ⚠️ Note: You don't need to do any of this in order to understand Hooks. Finally, we arrive at how Custom Hooks naturally arise. This will serve two purposes – to demonstrate the effective use of closures, and to show how you can build a Hooks clone in just 29 lines of readable JS. In this article, we reintroduce closures by building a tiny clone of React Hooks. However, their functional design requires a good understanding of closures in JavaScript. They were first introduced in React and have been broadly embraced by other frameworks like Vue, Svelte, and even adapted for general functional JS. Hooks are a fundamentally simpler way to encapsulate stateful behavior and side effects in user interfaces. Also, this article does not mention the React scheduler or how state is actually stored in React. Author's Note: This article has been turned into a talk, with more context.
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