Closure

What is a closure in JavaScript? Can you provide an example?

Ans: A closure is a function that has access to its parent function's scope, even after the parent function has returned. An example of a closure in JavaScript is when you create a function that returns another function that references a variable in the parent function's scope.

• variable stays around even after function returns
• when there's an inner function and an outer function
• a closure can make the creation of very similar functions efficient
• Closure functions can even modify bound variables in the background
• A closure is a function that has access to its outer function scope even after the outer function has returned. This means a closure can remember and access variables and arguments of its outer function even after the function has finished.
• A closure is the combination of a function and the lexical environment within which that function was declared. This environment consists of any local variables that were in-scope at the time the closure was created.
• a stateful function
• commonly used to give objects data privacy

function getAsyncData(callback){
     setTimeout(function(){
            callback("Here is the data")
      },3000)
}

var waitForAsyncData = new Promise(function(resolve, resolve){
     setTimeout(function(){
           resolve("gimme the light";
     })
});

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Closure Scope Chain

For every closure we have three scopes:

• Local Scope ( Own scope)
• Outer Functions Scope
• Global Scope

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1. Closures are useful because they let you associate some data (the lexical environment) with a function that operates on that data. This has obvious parallels to object-oriented programming, where objects allow us to associate some data (the object's properties) with one or more methods.

2. Consequently, you can use a closure anywhere that you might normally use an object with only a single method.

3. Situations where you might want to do this are particularly common on the web. Much of the code we write in front-end JavaScript is event-based — we define some behavior, then attach it to an event that is triggered by the user (such as a click or a keypress). Our code is generally attached as a callback: a single function which is executed in response to the event.